A . 5 p o ly M e r s High density and low density polyethene a molecule of high density polyethene (HDPE) The word “polymer” means “many parts”. Polymers (also called a molecule of low density polyethene (LDPE) plastics) are made up of repeating monomer units whose structures ▲ Figure 1 Little or no branching in the polymer can be manipulated in various ways to give materials with desired properties. Polyethene (sub-topic 10.2) is an addition polymer made of chain produces HDPE, which is stronger than ethene monomer units. The same monomer can be linked together to LDPE whose molecules are highly branched form high density polyethene (HDPE) or low density polyethene (LDPE), depending on the degree of branching in the polymer chain (gure 1). The branching in LDPE molecules makes the polymer more exible. HDPE can have M values of 200000 upwards. The linear structure of r the molecules allows for very close packing, improving the material’s strength which increases with weight. Ultra-high-molecular-weight polyethene (UHMWPE) can have M of 2–6 million; this is linear HDPE r of very high strength which shows resistance to cutting and abrasion and has been used in synthetic ice-skating rinks and to replace Kevlar in bullet-proof vests. HDPE and LDPE are produced from the same monomer using different methods and catalysts. LDPE is produced by free-radical polymerization involving an initiator, whereas a Ziegler–Natta catalyst is used to produce HDPE. (Knowledge of the mechanisms of these processes is not necessary for IB Chemistry.) Thermoplastics and thermosets Polymers can be classied as thermoplastics or thermoset plastics based on their behaviour when heated. Thermoplastics generally do not have straight molecules but rather are formed of a massive weave of polymers bound together by intermolecular (van der Waals’) forces that give them their shape. As a result they can be melted and then cooled in moulds to produce different shapes. The melting breaks down the intermolecular forces and on cooling new intermolecular forces form. Thermoset plastics are made by heating the raw materials ▲ Figure 2 HDPE is used for making bottles, like (monomers) and forming them into a single large network instead of the one pictured on the left. The water bottle many molecules. This results in a much stronger plastic because its on the right is made from another polymer, shape is held by covalent bonds rather than intermolecular forces. The polyethylene terephthalate (PETE) molecules may contain rings, linear chains, and side branches all bonded into one giant molecule. The structure cannot be melted and reformed into a different shape because melting would require sufcient heat to break the covalent bonds, hence decomposing the molecule rather than melting it. Thermoset plastic products are moulded when hot and they set as hardened plastic with the desired shape. They are harder, more rigid, and have higher strength than thermoplastics. Polyethene, polystyrene, polyvinyl chloride (PVC), and polypropene are some recyclable thermoplastics, whereas resins, epoxies, polyurethanes, Bakelite and polyesters are formed from thermosetting prepolymers into hardened thermosets. Figure 3 outlines the difference between thermoplastics and thermosets. 495
A M AT E R I A L S thermoplastics heat melt cool pm cool Plastics were vir tually unheard of prior to the Second World War. heat One of the earliest polymers was nylon, produced by DuPont. crosslinked thermoplastic thermoset polymer A technological advance in the thermosets heat/cure manufacturing of materials can dene an age, such as ▲ Figure 3 Thermoplastics have cross-links held by intermolecular forces and can be melted the Bronze Age and Iron Age. and reformed. Thermoset plastics are chemically bonded during formation and cannot Are we now in the “Polymer be reformed Age”? Using plastics we can quickly form raw materials into Elastomers many devices from medical applications to weapons. Elastomers are exible polymers that return to their original shape after How has the introduction of being deformed. They can be manufactured from either thermoplastics plastics aected the world or thermoset polymers but thermosets are usually chosen because of economically, socially, and their higher strength. When the material is not under stress the polymer environmentally? chain is tangled, loose, and exible. Under stress the molecules assume a more linear form but retain their shape afterwards due to the covalently bonded cross-links (gure 4). PVC and the use of plasticizers Polychloroethene or polyvinyl chloride (PVC) was discovered in 1835. It is formed from the monomer chloroethene, also called vinyl chloride: H Cl H Cl C C n C C n H H PVC chloroethene This addition polymer (sub-topic 10.2) was hard and brittle until Waldo Semon developed the technique of adding a plasticizer to the polymer to keep its strands somewhat separated. This reduces the intermolecular forces, softening the polymer and making it exible and durable. This more exible material had the added advantages of being water repellent and re resistant. ▲ Figure 4 An unstressed elastomer has tangled Plasticizers work by embedding themselves between polymer chains, long-chain strands (left) which straighten out thus reducing the intermolecular forces between these chains (gure5). when the elastomer is stretched (right). The This increases the volume, thereby lowering the density. The addition of covalent cross-links between polymer strands plasticizers also lowers the melting point and makes the material more provide strength and the elastomer will return exible and uid. One of the rst uses of plasticizers was to make PVC to the unstretched state once the stress is shower curtains. Plasticizing molecules such as bis(2-ethylhexyl)phthalate removed. Rubber is an elastomer (gure 5) contain both polar and non-polar groups. The polar group locks the plasticizer in the polymer and the non-polar group weakens some of the attractive forces in the polymer chain, thus enhancing exibility. Higher concentrations of plasticizer produce softer and more exible polymers. The plasticizer tends to evaporate over time so if exible PVC is left in a hot dry place for a long period the material will become brittle. The distinctive smell in a new car is associated with plasticizer evaporation. Plasticizers can be used to expand other materials such as concrete, but more than 90 % of plasticizer use is for polymers. 496
A . 5 p o ly M e r s H H H 2 2 2 C C C O CH C CH C 3 H 2 HC 2 O CH 3 O CH 3 HC C 2 O H CH CH 2 3 C key C C C plasticizer polymer chain H H H 2 2 2 bis(2-ethylhexyl) phthalate ▲ Figure 5 Plasticizers such as bis(2-ethylhexyl) phthalate contain polar and non-polar groups which allow them to embed between polymer chains, keeping them apar t and reducing the intermolecular forces Polystyrene H Polystyrene (polyphenylethene) is a thermoplastic polymer made C C from the monomer styrene (gure 6), a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical H H industry. Polystyrene can also be expanded, forming the lightweight and insulating expanded polystyrene familiar in food containers and styrene packaging (gure 7). It is produced from a mixture of polystyrene and a gaseous blowing agent (usually carbon dioxide, pentane, or another ▲ Figure 6 Phenylethene (styrene) volatile hydrocarbon). The solid plastic expands into a foam when heated by steam. Isotactic, atactic, and syndiotactic addition polymers Phenylethene undergoes addition polymerization similar to that shown by ethene in forming polyethene. Chloroethene (vinyl chloride), which polymerizes to polychloroethene (also known as polyvinylchloride, PVC), is another example of an addition polymer. Phenylethene and chloroethene can undergo several types of polymerization. These monomers are ethene with one hydrogen substituted by the phenyl group in phenylethene and by chlorine in chloroethene. Isotactic addition polymers have these substituents on the same side of the molecule, while atactic addition polymers have them randomly placed. Syndiotactic polymers have the substituents alternating one side to the next. Figure 8 shows these three forms in the case of polystyrene. Another important polymer is polypropene. This again has isotactic, syndiotactic, and atactic forms depending on the placement of the ▲ Figure 7 Polystyrene has three common forms. Solid/ex truded polystyrene has many methyl ( CH ) group. The isotactic form, with all the methyl groups on applications including models (pictured), 3 disposable cutlery and CD cases. Expanded polystyrene foam (shown below the model) the same side, is the most common commercial form. is used in packing materials and disposable cups. Ex truded polystyrene foam has good Isotactic propene is more compact than the atactic form, having a insulating proper ties making it impor tant as a regular repeating pattern that allows the molecules to come closer non-structural construction material together, increasing the van der Waals’ forces between them. Isotactic polypropene is harder, more rigid, and has a higher melting point than the atactic form. 497
A M AT E R I A L S C C C H C C H H C C C C H C H C C H C C C H C C H H C C C C H C C H C H H C H H C H H C H H C H C H C isotactic atactic syndiotactic ▲ Figure 8 Isotactic, atactic, and syndiotactic polystyrene (polyphenylethene) Syndiotactic polypropene has some stereoregularity: the regular, alternating placement of the CH groups allows closer packing and 3 stronger intermolecular forces than is the case for the atactic form, which is more amorphous and much softer than the isotactic or syndiotactic polymer. Atactic polypropene has weaker intermolecular forces; this hinders crystallization. Identifying monomers You need to be able to identify up to three repeating units in a polymer. For example, a monomer of 2-methylpropene can undergo addition polymerization (gure 9) by cleavage of the double bond. This polymerization produces butyl rubber, a synthetic rubber used during the second world war. The bond breaking is initiated by an acid + (H ) catalyst. CH 3 CH C 3 CH H CH CH 3 3 3 CH 2 H HC C C C HC C 3 2 CH 2 CH H CH CH 3 3 3 CH C 3 CH 3 CH CH CH CH 3 3 3 3 HC C CH C HC C etc. 3 2 2 2 CH CH CH CH 3 3 3 3 ▲ Figure 9 Addition polymerization of 2-methylpropene initiated by an acid catalyst 498
A . 5 p o ly M e r s H CH H CH CH H 3 3 3 The repeating unit with alternating methyl and hydrogen substituents C C C C C C is favoured over having two methyl substituents next to each other (gure 10). A lower energy path results if the bulky substituent H CH H CH CH H groups are not on neighbouring atoms in the polymer. 3 3 3 Atom economy of polymerization reactions ▲ Figure 10 The polymer conguration with alternating substituents (left) is favoured in the addition polymerization of 2-methylpropene In addition polymerization, all the reactant molecules (monomers) end up in the product which provides good atom economy (sub-topic 1.1). The atom economy is distinct from the percentage yield in that it is a measure of the mass of reactant molecules that end up in the desired product: molar mass of desired product ___ % atom economy = × 100% molar mass of all reactants For example, producing hydrogen by passing steam over coke may be a highly efcient process if all the reactants are converted to product which can be recovered, but this process does not represent good atomeconomy: C(s) + H O(g) → CO(g) + H (g) 2 2 The desired product, hydrogen, has M 2.02 while the total M of the 2.02 rr s reactants is 12.01 + 18.02 = 30.03. The atom economy is therefore The equation for atom economy is provided in the 30.03 Data booklet, which is available during examinations. ×100% = 6.7%, meaning that 93.3% of the mass of the reactants does not end up being in the desired product. Atom economy is a measure used in green chemistry, which takes into account not only the efciency but also the degree of waste produced. Efcient processes with high atom economy are important in sustainable development as they create less waste and use fewer resources. For example, ibuprofen was initially produced in a six-step process with an atom economy of about 40%. Research developed a three-step method which improved the atom economy to 77 %. The production of addition polymers represents 100% atom economy as all of the reactant monomer molecules end up in the product. Worked example theoretical yield __ Calculate the percentage yield (sub-topic 1.3) and Percentage yield = actual yield percentage atom economy if 1000 kg of iron(III) oxide, Fe O is reduced to 600 kg of iron by 600 kg 2 3 __ = × 100% carbon monoxide in a blast furnace: _6_9_.9_ × 1000 kg 100 Fe O (s) + 3CO(g) → 3CO (g) + 2Fe(l) 2 3 2 = 85.8% Atom economy Solution Theoretical yield = percentage of Fe in Fe O __2 × 5_5.85 _ = 2 3 × 100% 2 × 55.85 + 3 × 16.00 + 3 × 28.01 _2_× 55.85_ × 100% = 69.9% = = 45.8% 2 × 55.85 + 3 × 16.00 499
A M AT E R I A L S Questions 1 a) Many of the compounds produced by 5 The manufacture of low density poly(ethene) cracking are used in the manufacture of is carried out at very high pressures and at a addition polymers. State the essential temperature of about 500 K. A catalyst (either an structural feature of these compounds and organic peroxide or a trace of oxygen) is added to explain its importance. [2] the ethene. Explain how the catalyst reacts and write equations to show the mechanism of the b) The polymers often have other substances polymerization. [3] added to modify their properties. One group of additives are plasticizers. State how IB specimen paper, 2008 plasticizers modify the physical properties 6 Plastics, such as PVC and melamine resin, are of polyvinyl chloride and explain at the essential to modern society. molecular level how this is achieved. [2] a) PVC is thermoplastic whereas melamine resin IB, May 2011 is thermosetting. Explain how differences at a 2 During the formation of poly(styrene), a volatile molecular level affect the physical properties hydrocarbon such as pentane is often added. of these two types of polymer. [2] Describe how this affects the properties of the b) State one other way in which scientists have polymer and give one use for this product. [2] tried to classify plastics and outline why the IB, May 2010 classication you have chosen is useful. [2] 3 Addition polymers are extensively used in c) After its discovery it took almost a century society. The properties of addition polymers for PVC to be turned into a useful plastic, may be modied by the introduction of when Waldo Semon discovered the effect certain substances. of adding plasticisers. Explain how these affect the properties of PVC and how they a) For two different addition polymers, produce this effect. [2] describe and explain one way in which d) Justify why, in terms of atom economy, the properties of addition polymers may be the production of PVC could be considered modied. [4] “green chemistry”? [1] b) Use high-density poly(ethene) and e) In spite of the conclusion in D, many lowdensity poly(ethene) as examples to consider that the production of PVC is not explain the difference that branching can very environmentally friendly because its make to the properties of a polymer. [3] decomposition and combustion can lead to c) Discuss two advantages and two pollution. Identify one specic toxic chemical disadvantages of using poly(ethene). [2] released by the combustion of PVC. [1] IB, May 2010 IB specimen paper, 2013 4 Polyvinyl chloride (PVC) and polyethene are both polymers made from crude oil. a) Explain why PVC is less exible than polyethene. [2] b) State how PVC can be made more exible during its manufacture and explain the increase in exibility on a molecular level. [2] c) PVC can exist in isotactic and atactic forms. Draw the structure of the isotactic form showing a chain of at least six carbon atoms. [1] IB, November 2009 500
A . 6 n A not e c H nolo g y A .6 nah Understandings Applications and skills ➔ Molecular self-assembly is the bottom-up ➔ Distinguishing between physical and chemical assembly of nanopar ticles and can occur by techniques in manipulating atoms to form selectively attaching molecules to specic molecules. surfaces. Self-assembly can also occur ➔ Description of the structure and proper ties of spontaneously in solution. carbon nanotubes. ➔ Possible methods of producing nanotubes are ➔ Explanation of why an iner t gas, and not arc discharge, chemical vapour deposition oxygen, is necessary for CVD preparation of (CVD), and high pressure carbon monoxide carbon nanotubes. (HiPCO). ➔ Explanation of the production of carbon from ➔ Arc discharge involves either vaporizing the hydrocarbon solvents in arc discharge by surface of one of the carbon electrodes, or oxidation at the anode. discharging an arc through metal electrodes ➔ Deduction of equations for the production of submersed in a hydrocarbon solvent, which carbon atoms from HiPCO. forms a small rod-shaped deposit on the anode. ➔ Discussion of some implications and applications of nanotechnology. ➔ Explanation of why nanotubes are strong and good conductors of electricity. Nature of science ➔ Improvements in apparatus – high-power ➔ “ The principles of physics, as far as I can see, do electron microscopes have allowed for the study not speak against the possibility of manoeuvering of positioning of atoms. things atom by atom. It is not an attempt to violate any laws; it is something, in principle, that ➔ The need to regard theories as uncer tain – the can be done; but in practice, it has not been done role of trial and error in the development of because we are too big.” nanotubes and their associated theories. — Richard Feynman, Nobel Prize winner in Physics. What is nanotechnology? 9 “There’s plenty of room at the bottom” was the title of a 1959 talk by Richard 1 nm = 10 m Feynman proposing the feasibility of nanotechnology. Nanotechnology deals with the manipulation and control of atoms, molecules, and objects with dimensions of less than 100 nm (about 1000 atoms or less across). Chemical techniques place atoms in molecules using chemical reactions, whilst physical techniques allow atoms and molecules to be manipulated and positioned to specic requirements. There are two approaches to nanomanufacturing: top-down or bottom up. The top-down approach reduces large pieces of material down to the nanoscale. Optical lithography, for example, uses short wavelengths 501
A M AT E R I A L S of light (under 100 nm) in etching, such as in the design of integrated circuits. There is always some waste with the top-down approach as not all the material is used. The bottom-up approach uses molecular self-assembly of nanoparticles, in which molecules are selectively attached to specic surfaces. The principles of bimolecular recognition and self- ordering are used to build up particles in perfect order without any external driving forces. Examples of bimolecular recognition and self-ordering principles in molecular self-assembly include building up DNA strands via complementary base pairing, or other non covalently bonding principles like hydrogen bonding or metal coordination. The two key elements in molecular self-assembly are chemical complements and structural compatibility. Weak non-covalent interactions bind the substances together during the building process. Bottom-up molecular self-assembly produces more homogeneous nanostructures with less defects than results from the top-down process, largely due to the bimolecular recognition involved. Nanotubes One form of self-assembled nanoparticles is The main cylinder is made only from carbon hexagons, with pentagons needed to close the nanotubes, which are a type of fullerene structure at the ends. Theoretically, a wide range of shapes can be engineered at the molecular level molecule. Fullerene is an allotrope of carbon using fullerenes. Single-, double-, or multiple-walled nanotubes made from concentric nanotubes can be with atoms arranged into interlinking formed. Bundles of tubes have high tensile strength as strong covalent bonding extends along the hexagonal and pentagonal rings. Each carbon nanotube. The behaviour of electrons depends on the length of the tube; some forms are conductors is bonded to three rather than four other and others are semiconductors. Such structures have a wide range of technological and medical uses. 2 carbons, resulting in sp hybridized carbons, w h i c h c o n f e r s g o o d e l e c t r i c a l c o n d u c t i v i t y. Also, because all the carbon atoms are covalently bonded rather than held together by intermolecular forces, nanotubes are very strong. Constructing nanotubes Methods of producing nanotubes include arc discharge, chemical vapour deposition (CVD), and high pressure carbon monoxide disproportionation (HiPCO). Arc discharge was initially used to produce fullerenes, C , and involves either vaporizing the surface 60 of a carbon electrode or discharging an arc through metal electrodes submersed in a hydrocarbon solvent, forming a small rod-shaped deposit on the anode. Arc discharge using carbon electrodes Two carbon rods are placed about 1 mm apart in a container of inert gas (helium or argon) at low pressure. A direct current produces a high-temperature discharge between the two electrodes, vaporizing parts of one carbon anode and forming a small rod-shaped deposit on the other. The anode may be doped with small quantities of a 502
A . 6 n A not e c H nolo g y catalytic metal such as cobalt, nickel, yttrium, or molybdenum; in International collaboration in space this case single-walled nanotubes are formed. If pure graphite is used, exploration is growing. Would a carbon multi-walled nanotubes tend to be formed. Single-walled nanotubes nanotube space elevator be feasible? have a diameter of 0.5–7 nm whereas multi-walled nanotubes have What are the implications of such an concentric tubes with an inner wall diameter of 1.5–15 nm and outer advance, and would it be desirable? wall diameter of up to 30 nm (gure 1). low-pressure iner t gas graphite anode plasma discharge sooty deposit of carbon nanotubes graphite cathode vaccuum pump to maintain low pressure D.C. current single-walled nanotubes multi-walled nanotubes ▲ Figure 2 Computer image of a cylindrical fullerene rising from the ground to Ear th's orbit, acting as a space elevator. Such an elevator would allow people and materials to ascend and descend to and from space 10 nm 10 nm ▲ Figure 1 Arc discharge using carbon electrodes produces either single-walled or multi-walled nanotubes How do nanotubes grow? obtained using the chemical vapour deposition method. Because theories must accommodate There are several theories about the exact the assumptions and premise of other theories, mechanism of growth of nanotubes. For no universally accepted theory has yet been example, the diameter of nanotubes can vary formulated and trial and error plays a large part depending on the helium/argon concentrations. in this eld of research. The same catalyst can give different results using the arc-discharge method from those Arc discharge using metal electrodes Electrodes of a metal such as nickel can be used for discharge in a hydrocarbon solvent (gure 3), for example toluene (methylbenzene, C H ) or cyclohexane (C H ). The solvent is the source of carbon atoms 7 8 6 12 for the nanotubes as the hydrocarbon is decomposed by the arc and soot is produced either at the anode (as occurs with toluene) or dispersed hydrocarbon solvent metal (Ni) electrodes throughout the solvent. Chemical vapour deposition ▲ Figure 3 Experimental apparatus for arc discharge using metal electrodes and a In CVD gaseous carbon atoms are deposited onto a substrate. This is hydrocarbon solvent achieved by the decomposition of a hydrocarbon gas such as methane or ethyne, or carbon monoxide over a transition metal catalyst. The 503
A M At e r i A l s covalent bonds in the gas are broken by either plasma discharge or heat, cracking the molecule, and the carbon atoms diffuse towards a substrate which is coated with a catalyst. The catalyst is usually iron, nickel, or cobalt and is attached to the substrate by heating or etching. Once prepared the substrate is heated in an oven to over 600 °C and the hydrocarbon gas is slowly introduced. The gas decomposes and the carbon atoms reform into nanotubes on the substrate. The container must be free of oxygen or any other reactive substances to prevent the formation of carbon dioxide or any other impurities. The carbon atoms move to the substrate by diffusion and form either single- walled or multi-walled nanotubes depending on conditions. Either methane or carbon monoxide is heated to over 900 °C to form single- walled nanotubes while ethyne is heated to 600–700 °C for multi- walled nanotubes. Single-walled nanotubes have a higher enthalpy of formation than multi-walled nanotubes (sub-topic 5.1). quar tz boat quar tz tube gas inlet gas outlet CH N substrate with 2 catalyst 2 2 oven 720°C ▲ Figure 4 Chemical vapour deposition (CVD) One method of CVD is high pressure carbon monoxide disproportionation (HiPCO). In a disproportionation reaction the same substance is both oxidized and reduced. In HiPCO hot carbon monoxide is continuously supplied at high pressure into the reaction mixture. The catalyst iron pentacarbonyl, Fe(CO) is also fed in. The iron pentacarbonyl produces iron 5 nanoparticles that provide a nucleation surface for the reaction. No substrate is needed and the reaction can take place with a continuous feed making it suitable for industrial-scale production. In HiPCO carbon monoxide is reduced to carbon, which forms nanotubes, and is also oxidized to carbon dioxide: 2CO(g) Fe(CO) C(s) + CO (g) _____5 2 → As mentioned above, high temperature plasma rather than heating can be used to bring about CVD. A technique known as laser ablation uses a laser instead of an arc discharge to vaporize graphite. Either a continuous laser or pulses can be used and again single-, double-, or multi-walled nanotubes can be generated depending on conditions and the catalysts used. 504
A . 6 n A not e c H nolo g y Table 1 summarizes methods used to produce nanotubes. thq A ha la aba chma a Hh (cvd) ab mx a (Hpco) Mh Electric plasma Laser pulse strikes Uses heat to crack a Carbon atoms produced discharge vaporizes and vaporizes a graphite electrode, graphite gaseous hydrocarbon in the dispropor tionation depositing it on the into carbon atoms reaction from carbon other electrode as which are deposited on monoxide react with single- or multi-walled nanotubes a substrate containing vaporized Fe(CO) catalyst an etched-on transition 5 to produce nanotubes metal catalyst Aa In a hydrocarbon Continuous wave Plasma discharge Co–Mo catalyst instead of f h solvent using metal instead of pulse laser instead of heat Fe(CO) mh electrodes 5 s Iner t gas low-pressure Iner t gas low- Catalyst etched High pressure; T > 1,000 °C to crack hydrocarbon (lower atmosphere; ultra-pure pressure atmosphere; and deposited on with Co–Mo catalyst); graphite rods; T > gaseous ow; substrate; CH , C H temperature aects size of 4 2 2 single-walled nanotubes 3,000 °C; gap between T ~ 1,200 °C; graphite or CO; T > 1,000 °C to rods 1 mm or less powder or block/rods crack hydrocarbon or carbon monoxide y About 50% per batch; About 70% per batch About 50%; can Higher than 95% yield, can electrodes replaced before replacing produce large quantities run continuously with gas each time; about 10 g per day electrode; graphite (over 1,000 kg per day) ow, producing about 1 kg powder; less than 1 g due to continuous ow per day in a day and substrate size A aa Mostly defect-free Very high quality Easiest to scale up to Very high yields nanotubes single-walled industrial production nanotubes engineered to desired specications with ne control of diameter size da aa Small tubes with Very expensive Produces mostly multi- Some defects and random random sizes and directions, dicult to walled nanotubes with production purify many defects; dicult to separate single- from multi-walled nanotubes ▲ Table 1 Summary of methods of nanotube production CH → C+ H n m 2 CH C n m metal metal C C substrate substrate ▲ Figure 5 Nanotubes form on metal catalyst nanopar ticles. Two possible mechanisms are ex trusion from the substrate and catalyst or tip growth from the catalyst to substrate 505
A M AT E R I A L S Physical techniques in nanotechnology Scanning probe techniques tip is scanned relative to the sample AFM is used to measure friction between surfaces. (or sometimes the sample is scanned) Hair product manufacturers, for example, use AFM to study the effect of additives on hair at the tip measures some a feedback mechanism molecular level. AFM can also be used to measure proper ty of the surface is used to maintain the weak electrical forces on the surface of conductive tip at a constant height or semiconductive nanotubes. above the sample A distinct advantage of AFM is that it can be used ▲ Figure 6 Scanning probe techniques use various feedback in non-contact mode. The tip oscillates at a regular mechanisms to probe a nanosurface harmonic frequency and is brought to within a few nanometres of the surface. Intermolecular In the chemical processes of nanotechnology forces interfere with the oscillations and the production just described, carbon is vaporized resulting change in oscillations gives a picture of or obtained by cracking gaseous hydrocarbons the surface without contact. and fullerenes are allowed to reform into nanotubes. Physical techniques that manipulate materials at the molecular level include scanning probe microscopy techniques which can be used to probe and manipulate a molecular surface (gure 6). Atomic force microscopy Atomic force microscopy (AFM) is a scanning probe technique that uses a cantilever with a crystal tip of radius less than 10 nm made of microfabricated silicon or silicon nitride, Si N . 3 4 The cantilever is attracted or repelled either by ▲ Figure 8 Coloured atomic force micrograph (AFM) of contact or by interatomic van der Waals’ forces. A molecules of yttrium oxide, Y O on a thin lm of yttrium. 2 3 laser is reected off the tip and the reected beam Yttrium compounds are used in superconductors and lasers gives information about the surface. 4 quadrant photo detector A B cantilever deection measurement laser tip is in hard contact with the surface; repulsive regime ecrof tip is far from the surface; no deection xyz-stage AFM cantilever AFM sample stage sample 0 tip is pulled toward the surface – attractive regime probe distance from sample (z distance) ▲ Figure 7 Atomic force microscopy 506
A . 6 n A not e c H nolo g y Scanning tunnelling microscopy nanoscale. An electron beam from a scanning electron microscope is used to direct the There are many other forms of scanning probe synthesis of nanostructures, for example by microscopy. Scanning tunnelling microscopy HiPCO (gure 9). (STM) uses a metal tip with a small voltage applied to it to study electrical forces at the surface. An STM Fe(CO) electron beam image of a sample surface allows surface atoms to 5 scan be identied. The image is formed by moving a ne point just above the sample surface and electronically CO recording the height of the point as it scans. The electron clouds surrounding surface nuclei in the Fe point of the STM and the electron clouds in the sample surface overlap as they approach each other. ▲ Figure 9 Electron-beam-induced deposition An electric tunnelling current develops which can lead to an exchange of electrons. This rapid change of tunnelling current can be used to produce an image at the atomic level. This technique has applications in data storage and logic gates. Electron-beam-induced deposition Virtually any nanostructure shape can be produced quite accurately using this method. The technique of electron-beam-induced Small magnets, superconducting nanowires, and deposition is analogous to a 3-D printer on the nanogears are all theoretically possible. Implications and applications of nanotechnology Nanotechnology not only produces miniaturized uses have implications to some of today’s most products but also uses revolutionary molecular pressing problems, such as food shortages, manufacturing processes to make large products climate change, pollution, clean water, and from small machines. Nanomanufacturing has life-saving applications. the potential to produce life-saving medical applications and new products but also Nanotechnology also brings new problems. untraceable weapons of mass destruction. It There are health risks associated with enables the production of cheap, efcient, light, nanoparticles and their toxicity can vary strong structures including electrical and power depending on the size of the particles. How will storage equipment. the human immune system cope with particles on the nanoscale? How can the world control Molecular self-assembly does not require nano-weapons, which are easier and cheaper assembly lines and factories so once initiated it to build and less detectable than conventional can become an almost self-sustained process. weapons? As new materials and techniques are The technology has the potential to produce developed, regulations for their control need exponentially smaller computers which are faster to be developed. Materials that are safe on the and require less power. New stronger materials macro scale may not be safe on a nanoscale and at a fraction of the mass are being developed. nanoparticle waste products need new disposal Because molecular self-assembly works on methods. How should decisions regarding bimolecular recognition it has many possibilities funding be made, and by whom? for advances in medical applications. All these 507
A M AT E R I A L S Questions 1 Many recent developments in chemistry have 4 Nano-sized “test-tubes” with one open end can involved making use of devices that operate on be formed from carbon structures. a nanoscale. a) Describe these “test-tubes” with reference to a) (i) State the scale at which nanotechnology the structures of carbon allotropes. [2] takes place and outline the importance b) These tubes are believed to be stronger than of working at this scale. [2] steel. Explain the strength of these “test- (ii) State one public concern regarding the tubes” on a molecular level. [1] development of nanotechnology. [1] c) Carbon nanotubes can be used as catalysts. b) One development has been the production (i) Suggest two reasons why they are of nanotubes. Describe the way in which effective heterogeneous catalysts. [2] the arrangement of carbon atoms in the (ii) State one potential concern associated wall and sealed end of a nanotube differ. [2] with the use of carbon nanotubes. [1] IB, May 2011 IB, May 2011 2 Exciting developments have taken place in 5 Describe the chemical vapour deposition recent years in the area of nanotechnology. (CVD) method for the production of carbon a) Dene the term nanotechnology, and state nanotubes. [2] why it is of interest to chemists. [2] IB, Specimen paper 2013 b) Carbon nanotubes can be used to make 6 Outline what is meant by bimolecular recognition. designer catalysts Explain why it is essential for molecular self- (i) Describe the structure of carbon assembly. nanotubes. [2] 7 Explain why allotropes of carbon, graphene and (ii) State one physical property of carbon fullerene (used in producing nanotubes) are nanotubes. [1] conductive but diamond is not. c) Suggest two concerns about the use of nanotechnology. [2] IB, November 2010 3 Nanotechnology could provide new solutions for developing countries where basic services such as good health care, education, safe drinking water and reliable energy are often lacking. Discuss some of the potential risks associated with developing nanotechnology. [4] IB, May 2009 508
A .7 e n v ir on M e n tA l iM pA c t – pl A s t ics A .7 ema ma – a Understandings Applications and skills ➔ Plastics do not degrade easily because of their ➔ Deduction of the equation for any given strong covalent bonds. combustion reaction. ➔ Burning of polyvinyl chloride releases ➔ Discussion of why the recycling of polymers is dioxins, HCl gas, and incomplete hydrocarbon an energy-intensive process. combustion products. ➔ Discussion of the environmental impact of the ➔ Dioxins contain unsaturated six-member use of plastics. heterocyclic rings with two oxygen atoms, ➔ Comparison of the structures of polychlorinated usually in positions 1 and 4. biphenyls (PCBs) and dioxins. ➔ Chlorinated dioxins are hormone disrupting, ➔ Discussion of the health concerns of using leading to cellular and genetic damage. volatile plasticizers in polymer production. ➔ Plastics require more processing to be recycled ➔ Distinguish possible Resin Identication Codes than other materials. (RICs) of plastics from an IR spectrum. ➔ Plastics are recycled based on dierent resin types. Nature of science ➔ Risks and problems – scientic research often but the risks and implications also need to be considered. proceeds with perceived benets in mind, Challenges of materials science The oceans have rotating currents or , each with Green chemistry, also known as sustainable chemistry, is the design a calm spot at the centre. of chemical products and processes that reduce or eliminate the use Here oating plastic garbage or generation of hazardous substances. Green chemistry applies collects on such a scale that across the life cycle of a chemical product, including its design, the raft of plastic waste in the manufacture, and use. nor th Pacic gyre is estimated to be the size of Texas. How US Environmental Protection Agency should nations deal with the international problem of Although materials science has developed countless useful products, garbage in the oceans which it raises challenges associated with the recycling and toxicity of some aects the whole ecosystem? new materials. Plastics are polymers composed mainly of carbon and hydrogen. These have strong covalent bonds which are not easily broken so plastics do not decompose readily. Some polymers such as polyvinylchloride (PVC, polychloroethene) also contain chlorine and can release hydrogen chloride, HCl, or dioxins upon combustion. Other environmental concerns associated with plastics include the presence of volatile plasticizers. 509
A M AT E R I A L S The eect of plastic waste and POPs on wildlife Large plastic bottles and bags break down to incomplete combustion, producing carbon much smaller pieces in the ocean due to the monoxide and ne carbon soot particles. As action of the sun and abrasion by the waves. mentioned above, chlorinated compounds These smaller pieces can be mistaken for prey by such as PVC can release HCl gas and dioxins on marine animals. Over a million sea birds, marine combustion. The combustion reaction of the mammals, and turtles are killed each year from monomer chlorothene, for example, is given here: ingesting plastic. _1 Persistent organic pollutants (POPs) such as highly toxic dioxins can enter the food chain, CH =CHCl + 2 O → 2CO + HO + HCl having long-term effects on the health of animals 2 2 2 throughout the food chain. 2 2 ) gk gm( eussit yttaf ni sPOP Polar bear 1 1000 As POPs are passed along the food chain their 100 Seal concentrations increase and can reach very high levels in top predators (sub-topic B.6). This 10 Arctic cod process, known as biomagnication, has been 1 Zooplankton largely responsible for the extinction or signicant population reduction of many birds of prey and 0 large marine animals across the globe, including in regions far distant from the places where the 2 3 4 5 POPs were released to the environment (gure 1). trophic level (2 herbivores; 3–5 carnivores) Burning is not a viable means of waste disposal for plastics because polymers frequently undergo ▲ Figure 1 Biomagnication of persistent organic pollutants (POPs) in a food chain O Dioxins and PCBs O The name “dioxins” refers to a class of environmental pollutants that are ▲ Figure 2 Dioxins contain POPs. Dioxin molecules contain unsaturated six-membered heterocyclic rings with two oxygen atoms, usually in positions 1 and 4 (gure 2). The unsaturated six-membered most toxic member of this class is 2,3,7,8- tetrachlorodibenzodioxin (TCDD). heterocyclic rings with two oxygen atoms. This is 1,4-dioxin Certain dioxin-like polychlorinated biphenyls (PCBs) with similar toxic properties are sometimes included in the term “dioxins”. Over 400 types of dioxin-related compounds have been identied though only about 30 of these are considered to have signicant toxicity, with TCDD being the most toxic. Section 31 of the Data booklet gives the formulas of some representative dioxins (gure 3). O Cl n Cl Cl m m Cl polychlorinated dibenzofuran n polychlorinated biphenyls Cl O Cl O Cl m Cl O Cl O Cl n 2,3,7,8-tetrachlorodibenzodioxin polychlorinated dibenzo-p-dioxin ▲ Figure 3 Some examples of dioxin-related compounds 510
A .7 e n v ir on M e n tA l iM pA c t – pl A s t ics Polychlorinated biphenyls (PCBs) are synthetic organic molecules containing two benzene rings with some or all hydrogen atoms replaced by chlorine. Figure 3 shows that PCBs do not have a dioxin centre ring in their structure, but they have the same toxic effects as dioxins so are considered to be dioxin like. Figure 4 shows an example of a PCB. Cl Cl Cl Cl Cl s Cl Cl Cl The combustion of hydrocarbons releases ▲ Figure 4 An example of a PCB carbon dioxide and water on complete combustion. Chlorine- Dioxins are highly carcinogenic (cause cancer) and they accumulate in containing plastics can release fat tissue so their concentration increases up the food chain. According hydrogen chloride gas and to the World Health Organization more than 90 % of all dioxins found dioxins while sulfur-containing in humans come from food, mostly meat and dairy products or sh compounds can release sulfur and shellsh. The combustion of chlorinated plastics can also lead to dioxide. Given the reactants the production of dioxins. Dioxin-like substances act on a receptor and products you should be present in all cells and can cause reproductive and developmental able to balance the equation for problems. They damage the immune system and interfere with any combustion reaction. hormone action. Reducing the environmental eect: The international symbol for “Recycle, Reuse, Reduce” Plasticizers and chlorine-free plastics is a Mobius strip designed in the late 1960s (gure 5). Plasticizers such as phthalates are readily released into the Recycling of plastics can be environment because they are embedded in the plastic only by energy intensive. Should the intermolecular forces rather than by covalent bonds. As plastics age “Reuse, Reduce” components they release plasticizer molecules which can nd their way into of the symbol take on a biological systems by inhalation or ingestion. Whilst not as toxic as greater emphasis? Has the dioxins there is some evidence that they disrupt the endocrine system, use of this symbol increased affecting the release of hormones which leads to cellular and genetic environmental awareness? damage. Phthalates are now being replaced with less environmentally What factors inuence the harmful plasticizers. recognition of symbols? Chlorine-free plastics are also being used as substitutes for PVC. In the event of a house re such halogen-free plastics are less likely to release dioxins, HCl, or other toxic combustion products. Recycling of plastics Recycling rather than disposal is one of the most obvious ways of reducing environmental damage from any material. The atom economy increases while the need for the manufacture of new materials is reduced. Recycling of plastics, however, offers signicant challenges. Thermosets cannot be melted down and recycled. Heating chlorine-containing polymers carries the risk of releasing dioxins so the method of remoulding needs to take account of this. 511
A M AT E R I A L S Section 30 in the Data booklet Plastics are recycled based on their polymer type, identied by a resin provides a list of RICs. identication code (RIC). This coding system was developed by the Society of the Plastics Industry (SPI) in 1988. Its primary purpose was ▲ Figure 5 The international symbol the efcient identication of plastic polymer types, but it was soon for “Recycle, Reuse, Reduce” applied to the classication of plastics for recycling. The number on the code gives information about the polymer type rather than its hardness, how frequently it can be recycled, difculty in recycling, or colour. Recycling is an energy-intensive process. Plastic bottles for recycling need to be collected and separated from other material. The labels and any other debris are removed and the plastic is washed. It is automatically sorted using near-infrared scanning techniques and then manually checked again as incomplete sorting can lead to difculties with the process. The separated plastics are then ground into akes and any remaining water or debris is removed from the akes by centrifugation. The akes are then washed and dried again and any further foreign substances such as metals are removed. The recycled end product is not used in food containers as a safety precaution. Some plastics cannot be recycled into new products. For example, the plastic cases of some cell phones contain bromine which is a re retardant and these plastics cannot be put through a recycling process. The products and problems associated with recycling are summarized in table 1. r ia p A a r c (ric) ph Bottles for water and PET bottles can be rinsed and 1 hhaa (PET or other drinks including reused, especially as they do not PETE), also referred to PE TE as polyester, has high carbonated drinks, contain phthalate plasticizers. resistance to chemical dishwashing liquids, PET softens at about 80 °C so it is 2 solvents and makes a and food jars such as mechanically washed and crushed good barrier to gases and for peanut butter. Also HDPE liquids. It is clear and the used in carpet bres and for recycling. Dierent coloured resin can be spun into bottles are separated. threads or can make good microwave trays. optical surfaces. Hh- h Bottle caps, bottles HDPE is cleaned, shredded and (high-density polyethene, for milk , cosmetics, ground. It can be melted and HDPE) has high tensile and toiletries such as recycled for non-food plastic strength and is stier than shampoo, grocery and most plastics. It is usually trash bags, shipping applications such as plastic lumber opaque due to its high containers, hard hats, (timber-like mouldings) for decking density and can withstand buckets, recycling bins. and garden furniture, mouldings, high temperatures. HDPE is Injection moulding and bins. resistant to most solvents plastics for conduits, wire cable covering, and and relatively impermeable to gas and moisture. HDPE 3D printing. is widely used. 512
A .7 e n v ir on M e n tA l iM pA c t – pl A s t ics p h Both rigid and exible PVC contains chlorine and (polychloroethene, PVC) applications including plasticizers so should not be melted is resistant to grease and gaskets, gloves, or softened by heat. It can be reused chemicals and can be pipes, window frames, for a similar application, or formed used to produce a variety construction materials, into smaller items such as plastic 3 of shapes and strengths credit cards, clothing, and ties and binders. P VC due to the addition of spor ting equipment. Very dicult plastic to recycle: can plasticizers. Very stable to contaminate batches of recycled 4 corrosion and can be made Early uses included PET or HDPE. “vinyl” records and LDPE exible or sti. plastic shower cur tains. 5 lw- h Cling wrap and stretch LDPE is melted and turned into PP (low-density polyethene, lms, coatings inside plastic sheets which are then 6 LDPE) is tough, exible, and milk car tons and hot manufactured into other goods such PS transparent. Good barrier to and cold beverage cups, as envelopes, bin liners, tiles, lms 7 moisture. exible container lids. and sheets, carpets, and clothing. OTHER Injection mouldings, LDPE is not recycled into food adhesives, and sealants. containers. p Containers for yoghur t, PP is reused but not frequently (polypropene, PP) is a medicines, take-away recycled because of the need for thermoplastic polymer. It is meals, microwave accurate sor ting to be successful. strong, iner t, and resistant containers, bottle to acids and bases. PP tops and closures for Cleaned PP can be melted and remanufactured into various provides a good barrier to condiments. products. Recycled PP products moisture and oils. are often made by mixing virgin Also exible chairs, hinges, Its proper ties vary coat hangers, toilet seats, and recycled PP. Recycling alters depending on whether its and shing nets. the structure of PP so it can only be structure is isotactic or recycled a limited number of times. atactic. Can form bres as well as having some electrical applications. p can be rigid Styrofoam containers, Polystyrene is resistant to decay or foamed, both forms protective foam and is a major contributor to showing a fair degree of packaging, egg car tons, plastic waste in the nor th Pacic rigidity. plastic cutlery. gyre. It can be recycled as it is Expanded and extruded easily compressed and reblown; polystyrene have it can then be used in packaging. dierent uses. Polystyrene can be conver ted back to the styrene monomer in a continuous process, rather than melting and remoulding, but this process is energy intensive so it is more often compressed and reformed. This code is used if the Depending on type; eg Dependent on resin type and not resin is not one of the six large (20 litre) water usually commercially recycled. bottles. types above or if it is a mixture of the resin types. Polycarbonate is one such polymer under this code. ▲ Table 1 Plastic resin identication codes 513
A M AT E R I A L S Sorting plastics 1 While most plastics can be recycled the main (wavenumber 600-800 cm ) while the aromatic challenge in this process is sorting. One bottle C C, C=C bond in polystyrene will give a 1 of PVC material can pollute up to 100000 HDPE different absorption wavenumber (1500cm ) from 1 bottles if not separated, resulting in being melted an alkene C=C bond (1650 cm ). Reection can with them. Sorting by hand is cost prohibitive in be used to distinguish HDPE from LDPE. many cases. Scanning plastic bottles using infrared (IR) or near infrared spectroscopy can identify PVC the bonds in the molecules quickly. PVC, for other resins example, will show the characteristic C Cl bond Section 26 of the Data booklet gives information on other resins IR wavelength absorption. PVC feed conveyor spectrometer samples control unit air nozzle air reser voir ▲ Figure 6 Detecting the C–Cl bond in PVC by IR spectroscopy allows this plastic to be separated from other types 514
A .7 e n v ir on M e n tA l iM pA c t – pl A s t ics Questions 1 Scientic research often proceeds with 2 Outline why is PCB considered “dioxin like”, perceived benets in mind, such as the many but not a dioxin. uses of PVC, but the risks and implications 3 Atom economy is one of the key aspects of also need to be considered. green chemistry. a) Discuss, in terms of atom economy, a) Dene the meaning of “atom economy”. bond strength, and combustion b) Calculate the percentage atom economy of products whether the production of the following reaction if the target product PVC could be considered “green is N-methylphenylamine, C H NHCH : chemistry”? 6 5 3 C H NH + (CH O) CO → C H NHCH + b) Using sections 26 and 30 from the 6 5 2 3 2 6 5 3 CH OH + CO Data booklet, identify the structural 3 2 features of peaks A and B in gure 7 c) Dimethyl carbonate can be synthesized as and give the resin identication follows: code (RIC) of the plastic in question. 4CH OH + 2CO + O → 2(CH O) CO + 2H O 3 2 3 2 2 Explain your choice. 100 Suggest how the amounts of waste produced in the synthesis of N-methylphenylamine can be further reduced. 80 %/ecnattimsnart -3 4 LDPE has a specic gravity of 0.92 g cm and -3 60 HDPE has a density of 0.95 g cm . Suggest a reason why otation is not a good method of 40 separating these two plastics. 20 A 0 5 Many plastic materials are disposed of by combustion. State two disadvantages of B disposing of polyvinyl chloride in this way. 4000 3500 3000 2500 2000 1500 1000 6 Identify two difculties and two advantages in 1 wavenumber/cm recycling plastics. ▲ Figure 7 515
A M At e r i A l s A .8 s ma a X-a aah (AHl) Understandings Applications and skills ➔ Superconductors are materials that oer no ➔ Analysis of resistance versus temperature data resistance to electric currents below a critical for type 1 and type 2 superconductors. temperature. ➔ Explanation of superconductivity in terms of ➔ The Meissner eect is the ability of a Cooper pairs moving through a positive ion superconductor to create a mirror image magnetic lattice. eld of an external eld, thus expelling it. ➔ Deduction or construction of unit cell structures ➔ Resistance in metallic conductors is caused by from crystal structure information. collisions between electrons and positive ions ➔ Application of the Bragg equation, of the lattice. nλ = 2dsin θ, in metallic structures. ➔ The Bardeen–Cooper–Schrieer (BCS) theory ➔ Determination of the density of a pure metal explains that below the critical temperature from its atomic radii and crystal packing electrons in superconductors form Cooper pairs structure which move freely through the superconductor. ➔ Type 1 superconductors have sharp transitions to superconductivity whereas type 2 Nature of science superconductors have more gradual transitions. ➔ Impor tance of theories – superconducting ➔ X-ray diraction can be used to analyse materials, with zero electrical resistance structures of metallic and ionic compounds. below a cer tain temperature, provide a good ➔ Crystal lattices contain simple repeating unit cells. example of theories needing to be modied ➔ Atoms on faces and edges of unit cells are shared. to t new data. It is impor tant to understand the basic scientic principles behind modern ➔ The number of nearest neighbours of an atom/ instruments. ion is its coordination number. 400 Superconducting materials 300 Metals are good conductors of electricity because the metallic structure contains electrons that are free to move. As the thermal energy in metals Ω/ecnatsiser 200 increases, atoms in the lattice vibrate more and there are more collisions between electrons and ions. Some kinetic energy is converted to heat 100 with each collision. It is these collisions that are the cause of electrical resistance in metals, the resistance increasing with temperature (gure 1). 0 By decreasing the temperature there are fewer collisions, the electrons move in a more direct path, and the resistance is reduced: the 100 0 300 600 900 conductance of the material increases. 300 Superconductors are materials that offer no resistance to electric current temperature/K below a critical temperature. At very low temperatures many materials ▲ Figure 1 Resistance increases linearly with temperature for many conducting materials 516
A . 8 s u p e r c o n d u c t in g M e tA l s A n d X - r Ay c r ys tA llo gr A p H y ( A H l ) can exhibit this property. For some materials, at low temperatures energy 0.150 transfer becomes quantized rather than continuous – energy is exchanged in discrete bundles that have a minimum size. If that minimum size is not 0.125 Hg achieved then transfer does not occur, there is no loss of kinetic energy, and 0.100 the material becomes a superconductor at this critical temperature (gure 2). Ω/ecnatsiser 0.075 In 1933 Walther Meissner and Robert Ochenfeld found that superconducting materials will repel a magnetic eld. This is similar to 0.500 perfect diamagnetism (sub-topic A.2) in which all external elds are repelled; this is what occurs in a superconductor. The Meissner effect 0.025 T (gure 3) is the ability of a superconductor to create a mirror image of c an external magnetic eld, thus excluding it. When a magnet is brought 0 near the surface of a superconductor, the superconductor responds by 4.0 4.1 4.2 4.3 4.4 creating a magnetic eld that is the exact mirror image of the magnet’s eld. The superconductor behaves as an identical copy of the magnet temperature/K with like poles facing each other. When the magnet is removed from the superconductor the magnetic eld disappears. ▲ Figure 2 Superconductivity was rst obser ved in mercury, Hg in 1911. The mercury had to be near 4 K, the critical temperature for this substance, before the quantized eects of energy transfer were obser ved N S ▲ Figure 4 Demonstration of magnetic levitation as a result of the Meissner eect. A high- ▲ Figure 3 The Meissner eect. An ordinary temperature superconductor, yttrium–barium– copper oxide creates a mirror-image conductor (left) shows random electron magnetic eld of a small, cylindrical magnet. The magnet is oating freely above a movement and allows magnetic eld nitrogen-cooled, cylindrical specimen of a superconducting ceramic. The glowing vapour is penetration, whereas a superconductor liquid nitrogen, which maintains the ceramic within its superconducting temperature range (right) excludes any magnetic eld penetration, creating a mirror-image magnetic eld of any magnet brought near A landmark discovery Understanding how matter behaves at low different explanations from cold-temperature temperatures was a landmark in scientic research. superconductivity and theories explaining this Superconductivity at room temperature may need phenomenon are constantly evolving. Type 1 and type 2 superconductors We have seen that superconductivity is limited temperature is below T . The magnetic eld, B, c by the critical temperature, T , for the material. c also has a critical value B . Any eld strength c Above T , the superconducting properties and c larger than B will cause the material to revert Meissner effect are no longer exhibited. c Research into superconductors has shown that from superconducting to a normal conduction superconducting properties can also be disrupted band for that material. The value of B increases by sufciently high magnetic elds even if the c slightly as the temperature is lowered below T c 517
A M AT E R I A L S In the search for room-temperature (shows zero resistance); however if these superconductors, the superconducting properties magnetic vortices move then losses in of various alloys and ceramics as well as elements conductivity begin to occur. were examined. It was observed that as the magnetic eld strength is increased, materials In both types of material the current must remain behave in one of two ways. small because moving electrons create a magnetic eld. Most metallic elements that can superconduct ● A type 1 superconductor demonstrates below T are type 1 superconductors whereas a sharp transition from superconducting (showing the Meissner effect of expelling c magnetic elds) to normal behaviour (magnetic elds again penetrate the material alloys and metal oxide ceramics are largely type 2 and resistance returns to normal). superconductors. Type 2 superconductors have a higher critical temperature and can therefore act as superconductors at higher temperatures. ● A type 2 superconductor displays a range of Bardeen–Cooper–Schrieer properties with a gradual transition. It shows a (BCS) theory superconducting band when the temperature One of the rst theories to explain how superconductivity works on the molecular level is below T and when any external magnetic was developed by John Bardeen, Leon Cooper, c eld B, is also at a minimum. Above B but c below T the material exhibits zero resistance c but not perfect diamagnetism (it does not show superconducting state the Meissner effect) – some of the external + K magnetic eld can penetrate the material in a + K type of vortex. As long as the vortices remain + K in one location the material still superconducts e type 1 + K + K dle citengam + K B As a negatively charged electron passes between c the metal’s positively charged atoms in the lattice, the atoms are attracted inward. This distor tion of normal the lattice creates a region of enhanced positive charge which attracts another electron to the area superconducting range T superconducting state c temperature B type 2 area of distor tion c2 + + K + K K dle citengam normal e e mix ture of + Cooper pair normal and K superconducting + B + K c1 K T The two electrons, called Cooper pairs, become c locked together and will travel through the lattice. ▲ Figure 6 Lattice distor tion occurs in a wave-like manner temperature as a negative electron distor ts the lattice. This enhanced ▲ Figure 5 Type 1 superconductors exhibit a sharp transition region of positive charge attracts a second electron and from superconducting to normal behaviour above a critical the two pair up, forming a Cooper pair which will travel through the lattice together. Cooper pairs form and reform; temperature T and applied ex ternal magnetic eld B , they are responsible for superconductivity in type 1 superconductors c c whereas type 2 superconductors show a range of proper ties below T and above B c c 518
A . 8 s u p e r c o n d u c t in g M e tA l s A n d X - r Ay c r ys tA llo gr A p H y ( A H l ) and Robert Schrieffer. The Bardeen–Cooper– Schrieffer (BCS) theory explains that below the critical temperature electrons in superconductors form Cooper pairs which move freely through the superconductor. At low temperatures the positive ions in the lattice ▲ Figure 7 A funicular, like this one in Lisbon, has two cars are attracted to a passing electron, distorting the operating as a pair. As one car goes down it gives some of lattice slightly. A second electron is attracted to this its energy to the other car pulling it up. Less work needs to slight positive deformation and a coupling of these be done by the motor as gravitational potential energy is two electrons occurs. Such electron pairs, called transferred between them and the two cars form a pair with Cooper pairs, are not paired by the Pauli exclusion zero total momentum. In early funiculars water was placed in principle (sub-topic 2.2) and they behave differently the top car and emptied in the bottom one so that even less from single electrons (gure 6). In a Cooper pair any work needed to be done by the motor. Cooper pairs operate momentum that might be dissipated in a collision at zero total momentum with the lattice absorbing and re- between a single electron and the lattice is gained emitting phonons to the Cooper pair, allowing the pair to travel by the second electron. Any energy gained by the through the lattice unimpeded lattice from the rst electron propagates along the lattice in a wave-like motion called a phonon. The phonon is transferred to the second electron, and because phonons are quantized it transfers its entire bundle of energy. Because there is no loss of energy there is no resistance. It is as if the atoms of the lattice oscillate, creating slight positive and negative regions which push and pull the Cooper pair along. If the material is not cold enough the vibrational energy of the lattice is too great for phonon energy transfer, which is why superconductivity can only occur below a critical temperature. Applications of superconductors Ceramic materials, which are normally insulators, have become some New ways of thinking of the best high-temperature superconductors ( T > 138 K). Cuprate BCS theory can explain type 1 superconductors have blocks of alternating planes of atoms; for example, superconductivity but cannot explain the transition state of TlBa Ca Cu O has conducting CuO layers sandwiched between heavier type 2 superconductors or certain high temperature superconductors 2 2 3 9 2 termed “strange metals”. New ways of thinking, and perhaps even atom layers of BaO, TlO, and Ca. Experimenting with these layers in a paradigm shift in perceiving how matter behaves, may result from type 2 superconductors has raised the critical temperature, with (Tl Pb ) research into these materials. 5 2 519 Ba Mg Cu O demonstrating superconductivity properties above room 2 2 9 17 temperature. Superconductors have been used to detect small magnetic elds. Superconducting quantum interference devices (SQUIDs) can detect small changes in the tiny electromagnetic elds created by brain activity and are used in neural studies. SQUIDs are also used in submarines detecting undersea mines. Superconducting magnets are used in instruments such as magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machines as well as particle accelerators. Superconductors could theoretically aid electricity production, but a major challenge is that type 2 superconductors that can operate at higher temperatures are ceramic, so not suitable for making into wires and electrical components. Their material properties (sub-topic A.1) as well as their superconducting properties need to be considered.
A M AT E R I A L S Unit cells X-ray crystallography (sub-topic 21.1) enables analysis of the structures of crystalline substances. Type 2 superconductors have large complex crystal structures and knowledge of the arrangement of the atoms (ions) in their crystals can help explain their behaviour. Crystal lattices can be viewed as simple repeating unit cells, with atoms at the corners, faces, and edges of each cell shared with neighbouring cells. For example, sodium chloride, NaCl has a cubic crystalline structure in which each unit cell is a simple cube and each corner + atom is surrounded by six others: each Na ion is attracted to six neighbouring Cl ions and each Cl ion is attracted to six neighbouring + Na ions. The number of nearest neighbours for an atom in a lattice is its coordination number , and for a simple cubic structure such as NaCl the coordination number is 6. The unit cell is the simplest repeating pattern in a crystal. Different crystals form unit cells of many different shapes, including orthorhombic, hexagonal, and rhombohedral to name a few. However, in this topic we will consider only pure metals forming simple cubic, body centred cubic (BCC), and face centred cubic (FCC) unit cells (gure 8). simple cubic cell body centred cubic face centred cubic (BCC) (FCC) ▲ Figure 8 Cubic structures have all sides equal; length = width = height A simple cubic cell has eight atoms, one at each corner of the cell. A BCC cell has an additional atom in the centre of the cell making nine in total, whereas a FCC cell has the eight corner atoms plus an additional atom at the centre of each face of the cube, making 14 in total. In a simple cubic cell structure each atom forms the corner of not just one but eight cells. As mentioned above this structure has a coordination number 6, as each atom is in close contact with six others, but the unit cell itself is only equivalent to one atom (gure 9): _1 8 corners × atom per corner = 1 atom per unit cell 8 ▲ Figure 9 In this model of the simple cubic Like the simple cubic cell, the BCC cell has eight corner atoms each crystal structure the atom in the centre forms shared between eight unit cells, but the centre atom is not shared with a corner atom for eight dierent unit cells and any neighbouring cells. This cell is equivalent to 2 atoms: has a co-ordination number of 6 as it is equally attracted to its 6 nearest neighbours _1 (8 corners × atom per )corner + 1 central atom = 2 atoms per unit cell 8 520
A . 8 s u p e r c o n d u c t in g M e tA l s A n d X - r Ay c r ys tA llo gr A p H y ( A H l ) An FCC cell has eight corner atoms each shared between eight unit cells plus an atom at the centre of each cube face. Each face of a cell is shared with the neighbouring cell so each of the six face atoms represents half an atom per cell (gure 10). The 14 atoms contributing to an FCC cell make up a cell representing 4 atoms: _1 _1 atom (8 corners × atom per )corner + (6 faces × per )face = 8 2 4 atoms per unit cell In addition to the three structures discussed above with atoms on a face ▲ Figure 10 The atom at the face of a cube or a corner of a cell, atoms can also lie on the edge of a cell. An edge is shared between two unit cells, so atom is shared by four cells, as shown in gure 11. represents half an atom per cell ▲ Figure 11 An edge atom is shared by four unit cells. The representative number of atoms 1 per unit cell in the diagram on the left is 3: 12 edge atoms × atom per edge for each 4 Figure 12 summarizes the atom contributions in simple cubic, BCC, and FCC unit cells. simple cubic BCC FCC ▲ Figure 12 Simple cubic structures represent 1 atom per unit cell, BCC represents 2 atoms per unit cell and FCC 4 per unit cell The coordination number for a simple cubic unit cell is 6, as explained previously; for a BCC unit cell the coordination number is 8 as the centre atom is in contact with eight other atoms (see gure 12). An FCC unit cell has a coordination number of 12: each “face” atom is in contact with four corner atoms. It is also in contact with four face atoms from each of the two cells it is shared between. Some unit cell structures have a closer packing structure than others. The simple cubic structure with only 1/8 atom at each corner contains more open space than the BCC cell in which the space is lled with an additional atom. The FCC structure has the closest packing and metals with this type of unit cell are more dense than the 521
A M AT E R I A L S other two. Ta ble 1 s um m a r i ze s t he s i m pl e c u bic , BC C , a nd F C C c r yst al structures. u nmb f am nmb f am ca pa f m mb b am simple cubic body centred cubic 8; 1 at each corner 1 6 52% (BCC) 9; 1 at each corner 2 8 68% face centred cubic and a central atom (FCC) 14; 1 at each 4 12 74% corner and 1 on each face ▲ Table 1 Summary of the structure and proper ties of dierent metallic crystal structures Worked example The length of a cubic crystal edge can be determined ● density assuming simple cubic structure by X-ray diffraction, and this can be used to (1 atom per unit cell) = 1.15 × 10 23 g/4.32 × 23 3 3 determine the type of packing structure. In lithium, 10 cm = 0.266 g cm for example, the side length of the unit cell is ● density assuming BCC structure (2 atoms per 0.351 nm. The density of lithium is 0.535 g cm 3 . 23 23 3 g)/4.32 × 10 cm unit cell) = 2(1.15 × 10 Determine the packing structure for lithium. 3 = 0.532 g cm ● density assuming FCC structure (4 atoms per Solution 23 23 3 g)/4.32 × 10 cm unit cell) = 4(1.15 × 10 ● volume of cube = length × width × height = 3 = 1.06 g cm 9 3 3 29 3 m (0.351 × 10 ) m = 4.32 × 10 = 23 3 3 cm shows 4.32 × 10 ● The density of lithium at 0.535 g cm that lithium crystallizes in a BCC structure. ● mass of lithium atom = 6.94 g mol 1 /6.02 × 23 1 23 = 1.15 × 10 10 mol g Qk q Nickel has a density of 8.91 g cm 3 and X-ray diraction shows that the unit cell edge length is 0.3524 nm. Determine the packing structure for Ni. X-ray crystallography About 95% of all solids are crystalline. Metals form mainly cubic, BCC, FCC, and hexagonal close packing structures with a regularly repeating pattern. By reecting X-rays of known wavelength off different layers of the crystal the distance between the layers and hence the unit cell edge length can be determined. X-rays incident on a crystal are scattered in all directions. Either the wavelength of X-rays used or the angle of incidence of the X-rays can be adjusted until constructive interference occurs, when two light waves one layer apart bounce off the crystal in the same phase. This can then be used to determine the distance between these two layers. 522
A . 8 s u p e r c o n d u c t in g M e tA l s A n d X - r Ay c r ys tA llo gr A p H y ( A H l ) a θ θ θ n el d g θ le θ d d λ s 2 The Bragg equation is provided ▲ Figure 13 When light waves are reected from two layers in the same phase, constructive in section 1 of the Data booklet, interference occurs and a bright line appears as the two reected waves reinforce which will be available in the each other. The two light waves are an integral number of wavelengths, nλ, apar t when examination. constructive interference occurs Note that d and λ must have If the waves reected from the upper and lower layer are in phase, the same units, eg nm, pm. the wave travelling to the lower layer must have travelled an integer number of one-half wavelengths further to reach the lower layer and the same integer number of half-wavelengths to return to the top layer. Because crystals are arranged in a regular repeating pattern, repeated constructive interference between the layers reinforces the beam to a level where it is detectable. The distance between the layers in the crystal can be determined by the Bragg equation: nλ = 2dsin θ n is an integer, representing the number of wavelengths difference between the two reected X-rays; in a rst order diffraction pattern n = 1 wave, in a second order diffraction pattern n = 2 waves, etc. λ is the wavelength of the X-rays that gave the diffraction pattern d is the distance between two layers in the crystal θ is the angle of incident radiation to the crystal. Worked example Tantalum, Ta is a type 1 superconductor. When X-rays of wavelength 154 pm are directed at a crystal of Ta the rst order diffraction pattern is observed at 13.49°. Calculate the separation of the layers of atoms in the crystal and the density of Ta in g cm 3 given that it forms a BCC crystal. Solution nλ = 2dsin θ d = nλ/2sin θ = 154 pm/2sin (13.49°) = 330 pm 12 10 m = 330 × 10 length of unit cell = 330 pm = 330 × 10 cm 10 3 23 3 cm volume of unit cell = (330 × 10 cm) = 3.59 × 10 1 23 1 22 = 3.01 × 10 mass of one Ta atom = 180.95 g mol /6.02 × 10 mol g BCC structure contains 2 atoms per unit cell so: 22 23 3 3 g)/3.59 × 10 density = 2(3.01 × 10 cm = 16.8 g cm 523
A M AT E R I A L S Finding the atomic radius from X-ray crystallography data Atomic radii can be determined from the packing structure and the (2r) distance between atom layers as determined by X-ray crystallography. A ▲ Figure 14 For a simple cubic structure simple cubic cell has atoms touching as shown in gure 14. The length of the atomic radius is given by 2r a side of the cube, d, as determined by X-ray crystallography is therefore equal to the diameter of the atom, hence r = d 2 For an FCC structure the atoms touch along a diagonal, but not along the edge (gure 15). The diagonal represents 4 atomic radii, so using Pythagorean theorem: 2 2 2 2 2 (4r) =d +d or (4r) = 2d √2 d _ so r = 4 4r For a BCC structure the atoms touch along the diagonal of the cube rather than the diagonal of a face (gure 16). Pythagorean theorem d says that: d 2 2 2 2 (diagonal of cube body) = length + width + height ▲ Figure 15 For the FCC structure The cube diagonal is 4r (the diameter of the central atom plus the atomic radii of each corner atom), so since length = width = height = d: the atomic radius is given by 2 2 2 (4r) = d + d 2 2 (4r) = 3d √3 d _ r = 4 Table 2 summarizes these results. r u Am a d 4r simple cubic _d r body centred cubic (BCC) 2 r − d face centred cubic (FCC) _√ 3_d d d√ 2 4 _√ 2_d 4 ▲ Table 2 Atomic radius in terms of the length of the unit cell for dierent metallic crystal structures ▲ Figure 16 For the BCC structure the atomic 2 2 radius is given by (4r) = 3d Covalent and atomic radii and the functional use of the concept of atomic radius depends upon the context. The boundary The atomic radius calculated from X-ray of the outer electrons is not clearly dened and crystallography data assumes atoms are touching; depends on other interactions. this is equivalent to a covalent radius which may differ from the atomic radius given in section 9 of X-ray crystallography data take into account the the Data booklet. The idea of atoms as spheres with strong forces between atoms in a crystal. a xed volume is no longer an accepted model 524
A . 8 s u p e r c o n d u c t in g M e tA l s A n d X - r Ay c r ys tA llo gr A p H y ( A H l ) Worked example Determine the density of gold, in g cm 3 , if it has a FCC structure and an atomic radius of 144 pm. Solution For a FCC structure: √2 _ radius of atom r = unit cell length d × 4 _4r d= √2 = 4(144) 407.4 pm _ = 1.414 10 3 23 3 cm volume of unit cell = (407.4 × 10 cm) = 6.762 × 10 FCC unit cell has 4 atoms so: 1 3 196.97 g mol ____ density = 4 × = 19.4 g cm 23 1 23 3 × 6.762 × 10 cm 6.02 × 10 mol Questions 1 Superconductors are now widely employed (iii) Use sections 6 and 9 of the Data booklet in devices such as MRI scanners and MagLev to calculate the density of niobium in g 3 trains. Many superconductors involve cm niobium. a) Niobium is most commonly found in a (iv) Determine the atomic radius of niobium and explain why this value crystalline form having the cubic unit cell may differ from the one in the Data booklet shown in gure 17. c) The ground-state electron conguration for 4 1 niobium is [Kr]4d 5s . (i) Compare and contrast paramagnetic and diamagnetic materials and explain whether niobium is more likely to be paramagnetic or diamagnetic. ▲ Figure 17 (ii) Niobium exhibits type 1 superconducting properties at low Classify the crystal structure, the coordination temperatures when doped with number of the atoms and the number of atoms other materials. Sketch a graph that to which the unit cell is equivalent. illustrates type 1 superconductivity and explain how it is different from type 2 b) X-rays of wavelength λ =154 pm are superconductivity. diffracted from this crystal at an angle of 14.17 degrees. (iii) According to Bardeen–Cooper– Schrieffer (BCS) theory, Cooper pairs (i) Assuming n = 1, calculate the distance, confer superconductivity. Outline in pm, between layers of the crystal. how Cooper pairs are formed and the role of the positive ion lattice in their (ii) Use your answer from b)(i) to nd the formation at low temperatures. 3 volume of a niobium unit cell in cm 525
A M AT E R I A L S (iv) Electrical resistance has been viewed as a collision between conducting electrons and localized ▲ Figure 18 electrons in the lattice causing some loss of energy. There is a c) (i) Figure 18 shows a representative unit gradual decrease of resistance as materials get colder and electrons’ interactions lose strength. State what a paradigm shift is and justify why type 1 superconductivity and high temperature superconductivity could possibly require a paradigm shift. cell of chromium. How many chromium 2 The unit cells are shown for two ionic atom equivalents does the unit cell compounds, Q and R. contain? [3] (ii) Use appropriate data from the Data booklet and the information about the dimensions of the unit cell to calculate the density and atomic radius of chromium. (If you could not calculate an answer for part b), use a value of 250 pm, although this is not the correct value.) [2] Q R d) The solid circles (●) represent the metal ion 1 (M) and the open circles (O) represent negative 2 ions (X). 2 1 9 a) What is a unit cell? [1] b) Which analytical technique would 1 2 distinguish between the two compounds Q 2 1 and R? [1] c) Explain how the technique distinguishes the ▲ Figure 19 two compounds. [3] Figure 19 is an electron density map of 4-methylbenzoic acid obtained by X-ray d) Deduce the simplest formula of R and Q diffraction. from the unit cell. [2] IB, May 1998 (i) What must have been the physical state of the 3 When monochromatic X-rays are directed towards compound to obtain this map? [1] the surface of a crystal, some undergo diffraction. (ii) Which atoms in the molecule do not appear on this map? Why is this? [2] a) What is meant by the term monochromatic and why is this important in X-ray (iii) Comment on the electron density between crystallography? [2] atoms with reference to the type of bonding present. [1] b) When X-rays with a wavelength of 154 pm are directed at a crystal of chromium IB, November 2000 the rst order diffraction is found at 15.5 °. 4 Draw structures representing a face centred Calculate the separation of the layers of cubic and body centred cubic unit cell. 12 atoms in the crystal. (1 pm = 1.0 × 10 m) 5 Platinum has a lattice edge length of 392.42 pm [1] and crystallizes in a cubic rather than hexagonal form. 526
A . 8 s u p e r c o n d u c t in g M e tA l s A n d X - r Ay c r ys tA llo gr A p H y ( A H l ) a) Determine the expected diffraction 8 Copy and complete table 3. angle for a rst-order reection when em d lh ra ca f b f am monochromatic radiation of 0.1542 nm is 3 / m used. a fm X-a b) The density of platinum is 21.09 g cm 3 . / a Determine the packing structure of a unit cell. m aa/m 6 Sketch a graph of resistance versus temperature iron 125 BCC for a conductor and a superconductor. sodium 0.968 429 7 Deduce which part of gure 20 represents: platinum 21.09 FCC a) a normal conductor or a superconductor ▲ Table 3 above its critical temperature b) a type 1 superconductor exhibiting the Meissner effect below the critical temperature c) a type 2 superconductor showing mixed transition state? magnetic eld (a) (b) (c) ▲ Figure 20 527
A M At e r i A l s A .9 c a m (AHl) Understandings Applications and skills ➔ Condensation polymers require two functional ➔ Distinguishing between addition and groups on each monomer. condensation polymers. ➔ NH , HCl, and H O are possible products of 3 2 ➔ Completion and descriptions of equations to condensation reactions. show how condensation polymers are formed. ➔ Kevlar® is a polyamide with a strong and ➔ Deduction of the structures of polyamides and ordered structure. The hydrogen bonds polyesters from their respective monomers. between O and N can be broken with the use of ➔ Explanation of Kevlar ’s strength and its concentrated sulfuric acid. solubility in concentrated sulfuric acid. Nature of science ➔ Speculation – we have had the Stone Age, Bronze is the Age of Polymers, as science continues to manipulate matter for desired purposes? Age, and Iron Age. Is it possible that today’s age Condensation polymerization Condensation polymers are formed by a reaction that joins monomers and also produces small molecules as a condensation product. The formation of an ester from an alcohol and a carboxylic acid (sub-topic 10.2) is an example of a condensation reaction: as well as the ester, water is formed as the condensation product. In condensation polymerization, many monomers are joined by condensation reactions to form the polymer. For two monomers to be joined by condensation polymerization they must each contain two functional groups, for example, a dicarboxylic acid and a diol: O O C R + R´ ́ O O C R´ ́ n In the polyester product shown the carboxyl group on the left can react with a further alcohol molecule and the hydroxyl group on the right can react with a further carboxylic acid molecule, and so the polymer chain can continue to grow. 528
A . 9 c o n d e n s A t i o n p o ly M e r s ( A H l ) Instead of a dicarboxylic acid and a diol, the reaction may proceed with only one monomer that contains two functional groups: for example, 3-hydroxypentanoic acid contains both an OH group and a COOH group so can polymerize with itself: OH O OH CH 3 3-hydroxypentanoic acid monomer The esterication reaction Am m Although you will not be examined on the mechanism for esterication, Addition polymerization has you need to be aware that the reaction is acid catalysed and that it is the 100% atom economy because OH group from the acid and the H atom from the alcohol that join to form all the monomer ends up in the condensation product water: the desired product. This is not the case for condensation O O polymerization as the second condensation product is lost 3 from the polymer. HOCH + HO 3 2 + H R OH R OH R OCH 3 carboxylic acid tetrahedral ester intermediate Acyl chlorides are a class of organic compound in which the OH group of a carboxylic acid is replaced by a chlorine atom: R(C =O)Cl rather than R(C=O)OH. Acyl chlorides react with alcohols to form esters even more readily than do carboxylic acids. The condensation product is hydrogen chloride, HCl rather than water. The mechanism is the same and it is the chlorine atom that leaves the intermediate. As before, two functional groups are needed in the monomer; an example is hexanedioyl dichloride which reacts with a diol to form a polyester: O Cl Cl O hexanedioyl dichloride Acyl chlorides react with amines rather than alcohols in a condensation reaction that forms an amide. For example, ethanoyl chloride, CH COOH and methlyamine, CH NH react to form 3 3 2 N-methylethanamide, CH NHCOCH . A hydrogen from the amine 3 3 and the OH group from the acid condense to form hydrogen chloride: H H O C C H O H CH 3 H C + N C H HC N HCl 3 + H H Cl H ethanoic acid methlyamine methylethanamide hydrogen chloride 529
A M AT E R I A L S The mechanism is the same as for the esterication reaction except that there is an N rather than an O next to the carbonyl group forming an amide linkage. Polymerization again requires two functional groups permolecule. The building up of proteins from amino acid monomers is a type of n condensation polymerization. The amino acids contain two functional Nylon is a thermoplastic that was rst produced in 1935; it was one of groups: an amino group, NH and a carboxyl, COOH. The type of the rst synthetic bres. Nylon is a polyamide made by polymerizing 2 1,5-diaminopentane and decanedioic acid. protein formed depends on the number, type, and sequence of the Nylon-6,6 (gure 1) is made from amino acid monomers (sub-topic B.2). hexane-1,6-dioyl dichloride and 1,6-diaminohexane. This was the version Phenol–methanal plastics of the polymer that was produced commercially as it was cheaper and Phenol–methanal plastics are another example of condensation easier to use. polymers. The rst step in the reaction involves electrophilic substitution (see topic 20) of a hydrogen atom at the benzene ring with methanal: OH OH 1 CH OH 2 6 2 H 5 H + 3 4 phenol methanal The OH group in phenol is an ortho–para director, meaning that substitution of the hydrogen will occur on the number 2 (ortho) or number 4 (para) carbon atom in the benzene ring. The second part of the reaction is the condensation step: ▲ Figure 1 Molecular model of nylon: grey = CH CH carbon; white = hydrogen; blue = nitrogen; 2 2 red = oxygen. Notice the repeating amide linkages in this polymer and the acid and n + nH O amine ends that would allow this model to 2 continue to polymerize The reaction can continue with substitution occurring in either the 2- Electrophilic substitution of a hydrogen and/or the 4- position depending on the ratio of methanal to phenol. on phenol by methanal can occur next to the OH, on the number 2 or 6 carbon, Phenol–methanal polymers are thermoset plastics. They form resins called the or tho position. It could also and are used in laminates and adhesives. Because of their ability to occur opposite the OH, on the number withstand high temperatures and electric elds they are used as electrical 4 carbon, called the para position. insulators in construction and brake linings in vehicles. Alternatively it could occur on the number 3 or 5 carbon, called the meta position. Polyurethanes The OH on phenol is an or tho-para director, meaning substitution into the Polyurethanes are another type of condensation polymer, a polyamide, meta position is highly unlikely. Perhaps with a wide variety of uses: you'd like to investigate what makes a group on a benzene ring either an or tho- O O para or a meta director? C N C CH CH O H 2 2 n H 530
A . 9 c o n d e n s A t i o n p o ly M e r s ( A H l ) They form foams such as those used in padded chairs, elastomers used in paint, and bres to produce spandex (elastane), a synthetic fabric with elastic properties. Monomers used to form polyurethanes are often a diol or diamine and a dicyanate (cyanates have the N =C=O functional group). Modifying polymers PVC is modied by adding plasticizers to soften include rubber tyres which are not so temperature the material (sub-topic A.5). Another example sensitive and are elastomers. of polymer modication is blowing air through plastics to manufacture foams such as expanded Covalent bonds between polymer chains prevent polystyrene or padded polyurethane used in seat the chains from moving independently and cushions. Polymers can also be doped with a strengthen the elastomer. For example, Bakelite substance to add a desired property; for example, is a phenol–methanal polymer that has cross- polyethene may be doped with iodine to increase linking between the 2- and 4-positions in the its conductivity. Fibres are also blended for comfort. benzene ring. This cross-linking makes Bakelite strong, rigid, and resistant to heat: The same chemical backbone, polyurethane for n n example, can be modied to form elastomers and adhesives, high-density material such as rubber + H (polymerize) soles for shoes, or padded cushions by air injection. H HO HO Other ways of modifying the properties of polymers phenol methanal include changing the polymer chain length; for Bakelite example, having more CH units in the molecule 2 OH increases the melting point as larger molecules have stronger intermolecular forces. The orientation of n substituent groups also has an inuence. The trans orientation of functional groups such as that seen in Kevlar (see sub-topic A.4, gure 11) allows close The degree of branching of the chain also inuences polymer properties. HDPE, for example, approach of the polymer chains and increases the has linear chains with little branching while LDPE has highly branched chains (sub-topic A.5). degree of hydrogen bonding between the chains, conferring strength to the polymer. Isotactic and atactic orientation in addition polymers were explained in sub-topic A.5. Ion interaction can also alter polymer properties. (a) (b) Ion implantation involves bombarding the polymer with large numbers of ions. This process (c) (d) can selectively modify the surface without changing the material’s bulk properties, for ▲ Figure 2 (a) Linear molecules can pack close together, example to increase or reduce friction. The ions eg HDPE. (b) Branched polymers are less dense and are held can interact with polar ends of polymers and together by weak intermolecular forces, eg LDPE and natural increase intermolecular forces. If negative ions rubber. (c) Cross-linking in polymers involves covalent bonds are added this allows metal complexes to form. joining polymer chains to each other and greatly increases Cross-linking between polymer chains can add strength, eg vulcanized rubber. (d) Networked polymers, such strength to elastomers. Rubber, for example, is as Bakelite and epoxy resins, are par ticularly strong and rigid a natural polymer which can be vulcanized in a process that adds sulfur to the polymer which creates strong covalent bonds between polymer chains. Natural rubber is soft and temperature sensitive, being brittle when cold and deforming easily when warm. Vulcanized rubber products 531
A M AT E R I A L S sa pha exam cha h The longer the chain, the stronger the Longer polymer chains have higher polymer. melting point, increased strength, and increased impact resistance due to increased van der Waals’ forces. Bah a ak Straight unbranched chains can pack HDPE with no branching is more rigid more closely. A higher degree of than the more branched LDPE. Use of plasticizers in PVC to soften the branching keeps strands apar t and polymer. weakens intermolecular forces. s mm Hydrogen bonding can increase Polystyrene strength, eg Kevlar. Atactic and Vulcanized rubber, Bakelite c-k ▲ Table 1 Summary of polymer proper ties isotactic placement can inuence strength, eg polystyrene. Extensive covalently bonded cross- linkage increases polymer strength. Breaking down condensation polymers Condensation polymers are formed from two monomers, releasing a small molecule in the process. These polymers can be broken down by the reverse reaction. Proteins, for example, are hydrolysed (a reaction that adds water) to amino acids during digestion. Polyamides with strong hydrogen bonding such as Kevlar can dissolve in sulfuric acid: the acid donates a proton to the oxygen and nitrogen atoms involved in hydrogen bonding. This breaks hydrogen bonds between chains of Kevlar bres and the substance dissolves. O H H O N O O H H O N H N O N N H H O ▲ Figure 3 Strong hydrogen bonds between polymer chains in Kevlar. Care must be taken to avoid interfering with hydrogen bond formation during production; for example the solvents must be free of ion impurities Nylon, another polyamide, reacts readily with dilute acids in a hydrolysis reaction. The amide linkages in Kevlar are somewhat more resistant to acid attack than is nylon, but acids break the hydrogen bonds reducing the strength of the polymer. In breaking down amides to amines and carboxylic acids the condensation product, water, must be added and the reaction proceeds faster at high temperatures. Steam at a pH much + greater or less than 7 can be used to break down a polyamide as an H or OH ion will initiate the hydrolysis reaction. 532
A . 9 c o n d e n s A t i o n p o ly M e r s ( A H l ) Questions 1 Which pair of compounds can be used to 4 Polymers, used extensively worldwide, are prepare CH COOCH ? large molecular mass substances consisting of 3 3 repeating monomer units. A. Ethanol and methanoic acid a) Distinguish between addition and B. Methanol and ethanoic acid condensation polymers in terms of how the C. Ethanol and ethanoic acid monomers react together. [2] D. Methanol and methanoic acid [1] b) Describe and explain how the properties IB, November 2006 of condensation polymers depend on three 2 Nylon is a condensation polymer made up of structural features. [3] hexanedioic acid and 1,6-diaminohexane. IB, May 2009 Which type of linkage is present in nylon? 5 a) Kevlar can be made by reacting 1,4-diaminobenzene, H NC H NH , A. Amide 2 6 4 2 with 1,4-benzenedicarbonyl chloride, B. Ester ClOCC H COCl. Write the equation 6 4 C. Amine for the reaction of n molecules of D. Carboxyl [1] 1,4-diaminobenzene reacting with n molecules of 1,4-benzenedicarbonyl chloride. IB, May 2007 [2] 3 Kevlar is a condensation polymer that is often IB, May 2010 used in liquid-crystal displays. A section of the polymer is shown in gure 4. H H H N N C C N O O ▲ Figure 4 a) Explain the strength of Kevlar in terms of its structure and bonding. [2] b) Explain why a bullet-proof vest made of Kevlar should be stored away from acids. [2] IB, May 2011 533
A M At e r i A l s A .10 ema ma – ha ma (AHl) Understandings Applications and skills ➔ Toxic doses of transition metals can disturb ➔ Explanation of how chelating substances can the normal oxidation/reduction balance in cells be used to remove heavy metals. through various mechanisms. ➔ Deduction of the number of coordinate bonds a ➔ Some methods of removing heavy metals are ligand can form with a central metal ion. precipitation, adsorption, and chelation. ➔ Calculations involving K as an application of sp ➔ Polydentate ligands form more stable removing metals in solution. complexes than similar monodentate ligands ➔ Compare and contrast the Fenton and Haber– due to the chelate eect, which can be Weiss reaction mechanisms. explained by considering entropy changes. Nature of science ➔ Risks and problems – scientic research often but the risks and implications also need to be considered. proceeds with perceived benets in mind, Applications of heavy metals Toxic metals can react with enzyme binding sites and inhibit or over-stimulate these enzymes. For “Heavy metals” is a term that refers to toxic example, cadmium belongs to the same group as metals such as lead, mercury, and cadmium zinc, and competes with zinc during absorption which have cumulative effects on health. into the body. Lead can compete with and replace Such metals have many uses: lead, nickel, and calcium in much the same way. Even when we cadmium are used in batteries; arsenic, bismuth, take in more zinc and calcium in the diet, the toxic and antimony are often found in semiconductors; metals are not eliminated and tend to accumulate. and mercury has many uses including in instruments such as thermometers, barometers, Toxic doses of transition metals can disturb the and diffusion pumps and has been used in normal oxidation–reduction balance in cells mining, amalgams, and manufacturing. Heavy through various mechanisms. They can disrupt metals are commonly used as catalysts and have the endocrine system because they compete for historical uses such as lead for pipes, lead paint, active sites of enzymes and cellular receptors. They and petrol additives. exhibit multiple oxidation states so can participate in redox reactions, and they can initiate (free) Heavy metals accumulate in biological systems radical reactions in electron transfer. Their ability over time. They are stored in living organisms and to form complex ions enables them to bind with passed on in the food chain (see biomagnication enzymes: iron, for example, forms a complex in sub-topic B.6). The toxicityand carcinogenic with hemoglobin which is essential for oxygen properties of heavy metals are the result of their transport. Finally, transition metals are very good ability to form coordinated compounds, exist in catalysts (topic 13). various oxidation states, and act as catalysts in the human body. 534
A . 10 e n v i r o n M e n t A l i M p A c t – H e A v y M e t A l s ( A H l ) Haber–Weiss and Fenton reactions Free-radicals (sub-topic 10.2) can be generated Notice that in accordance with Hess’s law, the two steps of the Fenton reaction result in the Haber– naturally in biological systems; for example, the Weiss reaction. superoxide free-radical ion, O is a product of cell 2 metabolism. The Haber–Weiss reaction offers an The highly reactive OH radical is one of the explanation of how a more toxic hydroxyl radical, most damaging free-radicals in the body. It OH, could be formed. It was recognized that transition reacts with almost any molecule it encounters metals can catalyse this reaction, with the iron- including macromolecules such as DNA, catalysed (Fenton) reaction providing a mechanism for membrane lipids, and enzymes. Because it is so generating these reactive hydroxyl radicals. reactive it can be used to break down pollutants The Haber–Weiss reaction is a slow process that such as pesticides and phenols and the Fenton generates hydroxyl radicals, OH, from hydrogen reaction is carried out in waste-water treatment peroxide and the superoxide free-radical ion, O : plants. For example, benzene derivatives, which 2 are not very reactive, can be oxidized to less O + HO → O + OH + ·OH 2 2 2 2 toxic phenols: The products include a hydroxide ion as well as a 2 OH + CH → C H OH + HO hydroxyl radical. The peroxide reactant is formed 2 by certain enzymes acting on the superoxide free- 6 6 6 5 radical to catalyse a disproportionation reaction: The OH radical created by the Fenton reaction is a rst step in many industrial processes. It can be used to eliminate some greenhouse 2 O + + → O + HO 2 2H 2 2 2 gases such as methane from plant emissions, The peroxide–superoxide reaction is much and to reduce odour from waste-water quicker when catalysed in a two-step reaction, the Fenton reaction: treatment sites. The highly reactive radical can break C=C double bonds, open up aromatic 3+ 2+ rings, degrade hydrocarbons, and even initiate Fe Fe + ·O → + O 2 2 polymerization. 2+ + HO → 3+ + ·OH + OH Fe Fe 2 2 gba ma f ah O Fritz Haber is best known for xing nitrogen (synthesizing ammonia), and he received the Nobel Prize in Chemistry in 1918 for this work . His nal paper in O O 1934 proposed that the reactive hydroxyl radical could be generated from the superoxide ion and hydrogen peroxide. This greatly enhanced understanding of N O the role of radicals in biochemistry. N O Haber ’s synthesis of ammonia for fer tilizers enabled mass food production, alleviating hunger. It is ironic that his research in chemical warfare went side by side with this. The ethics of scientic research have global implications. M O O Chelating eects O Apart from the Fenton reaction, other methods of removing heavy metals ▲ Figure 1 EDTA is a polydentate include precipitation, adsorption, and chelation. Chelation takes advantage ligand that can form up to six of a metal’s ability to form complex ions. The word “chelate” is derived from coordinate bonds to a central metal the Greek for “claw” and refers to polydentate ligands (sub-topic 13.2; some ion. It is used in chelation therapy common polydentate ligands are given in section 16 of the Data booklet). to treat lead poisoning and remove Chelating agents are used to remove heavy metals such as lead, arsenic, and excess iron from the blood of mercury from the body. Once chelated the complex ion is too large to enter patients with thalassemia cells but being an ion is water soluble so can be excreted from the body. 535
A M AT E R I A L S To act as ligands, chelating agents must have lone pairs of electrons that can form coordinate covalent bonds to a central atom. “Polydentate” refers to their ability to form more than one such coordinate bond. Figure 1 shows that EDTA can form two, four, or up to six coordinate covalent bonds with a central atom. Ethylenediamine (ethane-1,2-diamine) is a bidentate chelating agent: ̈ CH CH ̈ HN 2 2 NH 2 2 Heme in hemoglobin forms four coordinate covalent bonds to iron. dma a ma a H H H S C ▲ Figure 2 Workers at a heavy metal recycling H H O H factory. ‘Heavy metals’ refers to toxic metals S C such as lead, mercury, and cadmium which have C accumulative eects. Heavy metals not recycled H must be carefully disposed of in toxic landll sites H ▲ Figure 3 Dimercaptol is a bidentate chelating agent that uses the lone pairs of electrons on its two sulfur atoms to form coordinate bonds with mercury, arsenic, antimony, and gold. Dimercaptol was used to treat arsenic-containing mustard gas during the rst world war. Chelated metals cannot enter cells and can be excreted from the body Polydentate ligands such as EDTA are usually more effective than monodentate ones and will replace them in reactions. Competition in ligands was discussed in topic 13, and one factor inuencing this is the increase in entropy involved. Nickel, for example, can form a complex ion 2+ with six molecules of water [Ni(H O) ] . EDTA will replace the six water 2 6 molecules in this reaction forming a larger complex and releasing the six smaller molecules thus increasing the overall entropy: 4 2+ 2 EDTA [Ni(EDTA)] (aq) + [Ni(H O) ] (aq) → (aq) + 6H O(l) 2 2 6 ▲ Figure 4 An industrial waste treatment plant in The existence of a greater number of smaller molecules rather than Argentina. Toxins are removed and the water one larger one yields more ways of distributing the effective energy, puried before being put back into the environment. and hence represents an increase in entropy. This is one reason why Water treatment is needed in many places as many chelation is effective at removing metals, as the polydentate ligand will of the world's major rivers show high pollution replace larger numbers of existing ligands, usually water. Solubility product constant, K sp + + Metal ions from group 1, including K , Li , and hydroxides are only slightly soluble so hydroxide + ions are often added to precipitate the metal ions Na , form highly soluble compounds whereas the heavy metal ions generally form compounds as the level of hydroxide ions can be monitored by of low solubility. Their salts precipitate easily and measuring the pH. Lime, Ca(OH) , is commonly this means heavy metal ions can be removed 2 used as it is a relatively cheap and abundant during waste-water treatment. Many heavy metal material. 536
A . 10 e n v i r o n M e n t A l i M p A c t – H e A v y M e t A l s ( A H l ) 2+ The solubility of metal ions can be expressed as The concentration of Pb is the same as the the position of equilibrium of the solid salt with molarity of Pb(OH) , so the molar solubility of 2 7 3 mol its aqueous ions. For example, the equilibrium lead(II) hydroxide is 1.53 × 10 dm expression can be written for the highly insoluble lead(II) hydroxide, Pb(OH) : Example 2 2 2+ Pb(OH) (s) ⇋ Pb (aq) + 2OH (aq) Cadmium is a heavy metal frequently removed 2 from waste water by precipitation. The water The position of equilibrium at standard conditions is adjusted to pH 11 by adding lime (calcium (at 298) can be expressed as a constant (topic 7). hydroxide). Calculate the molar solubility of the Solids are not included in equilibrium expressions 2+ Cd ion at this pH. as they have zero concentration in the solution. This particular equilibrium constant is referred to Solution as the solubility product constant , K and the sp value of K for Pb(OH) at 298 K is 1.43 × 10 20 A K [Cd(OH) ] = 7.2 × 10 15 . (from section 32 sp 2 sp 2 table of solubility product constants can be found of the Data booklet) in the Data booklet. 3 At pH 11, [OH ] = 10 2+ Worked examples Cd(OH) (s) → Cd (aq) + 2OH (aq) 2 2+ [Cd ] [OH ] Example 1 3 3 I0 10 of lead(II) Calculate the solubility in mol dm hydroxide. 3 + 2x C +x 10 E x 10 3 + 2x Solution 15 3 2 = x(10 7.2 × 10 + 2x) . Because the degree of Using the ICE method as explained in topic 17: dissociation of Cd(OH) is small compared with the 2 2+ Pb(OH) (s) ⇋ Pb (aq) + 2OH (aq) 3 2 concentration of the hydroxide 0.001 mol dm 2+ 2 ion, the + 2x in the [OH ] term can be ignored. ] K = [Pb ][OH sp 15 3 2 6 = x(10 ) 7.2 × 10 ) = x(10 I 0 0 9 3 mol x = 7.2 × 10 dm C +x +2x 2+ Notice the low solubility of Cd ions at this E x 2x pH. See if you can conrm for yourself that the 20 2 3 1.43 × 10 = (x)(2x) = 4x 2+ solubility of Cd without adjusting the pH is 1.2 7 5 , or about 10000 times higher. x = 1.53 × 10 × 10 Adsorption of heavy metals Activated charcoal is an expensive adsorbent. Cheaper Another method of removing heavy metals is by adsorption onto a solid agricultural methods are surface. There are many methods including activated carbon, charcoal proving useful in many lters and clays. Biomass such as brewer’s yeast has also been found developing countries. Coconut to be effective. Ion-exchange mechanisms which exchange heavy shells, rice husks, and sugar metal ions for calcium or sodium ions can also remove heavy metal cane have adsorbent proper ties contaminants. The treated water then undergoes further purication which might be eective in processes such as ultraviolet treatment to kill bacteria. removing heavy metals. 537
A M AT E R I A L S Questions 1 Hydroxyl free-radicals can be generated c) Magnesium ion concentrations can be naturally in the body. This process is catalysed determined by precipitation as magnesium by iron in the following two steps: hydroxide. Given that the solubility 3+ 2+ product, K , of magnesium hydroxide is Fe Fe sp reaction 1: + ·O → + O 2 2 11 1.20 × 10 calculate the concentration, in reaction 2: 2+ + HO → 3+ + ·OH + OH mol dm 3 Fe Fe , of magnesium ions required to 2 2 form a precipitate in a solution where a) Use the above information to write the the nal hydroxide ion concentration is uncatalysed reaction. 3 2.00 mol dm b) Deduce whether iron is acting as a heterogeneous or homogeneous catalyst and justify your answer. 3 Chromium(III) ions form a hexa-aqua c) Show that reactions 1 and 2 are redox complex ion, [Cr(H O) 3+ Write a ] (aq). 2 6 equations by writing the oxidation half- balanced equation for the reaction of this equation and the reduction half-equation 4 (aq) and explain why complex with EDTA for each. EDTA will replace the water in the complex ion. 2 Heavy metals are often removed from solutions by precipitation. 4 Explain the difference between precipitation, chelation, and adsorption as methods of a) Use section 32 of the Data booklet to removing heavy metal contamination. calculate the concentration of sulfuric acid necessary to precipitate mercury(I) ions at a 3 concentration of 3 µmol dm 5 Use section 32 of the Data booklet to calculate b) Evaluate the effectiveness of this method the molar solubility of zinc at pH 11 and for mercury removal and suggest explain why zinc ions are more soluble in acidic improvements. solutions. 538
B BIOCHEMISTRY Introduction of matter. The processes in the living cells resemble the reactions of traditional chemistry Biochemistry studies chemical processes in and therefore can be studied and replicated living organisms at the molecular level. Despite in the laboratory or utilized in industry, the diversity of life forms and complexity of agriculture, and medicine. Biochemical biological structures, life functions can be studies enhance our understanding of the interpreted in chemical terms, because the phenomenon of life and our own place in the constitution and properties of biomolecules natural world. are governed by the same principles as the constitution and properties of any other form B.1 Itoctio to biocmit Understandings Applications and skills ➔ Shapes and structures of biomolecules dene ➔ Deduce condensation and hydrolysis reactions their functions. and explain the dierence between these ➔ Metabolic processes take place in aqueous processes. solutions in a narrow range of pH and temperature. ➔ Describe the balancing of carbon and oxygen ➔ Anabolism is the biosynthesis of complex in the atmosphere by summary equations of molecules from simpler units that requires energy. photosynthesis and respiration. ➔ Catabolism is the biological breakdown of complex molecules that provides energy for living organisms. Nature of science ➔ Condensation reactions produce biopolymers ➔ Biochemical systems are complex and involve that can be hydrolysed into monomers. a large number of simultaneous chemical reactions. The development of analytical ➔ Photosynthesis transforms light energy techniques allows us to collect enough into chemical energy of organic molecules experimental data to reveal cer tain patterns synthesized from carbon dioxide and water. in biochemical processes and eventually ➔ Respiration is a set of catabolic processes that understand metabolic processes. produce carbon dioxide and water from organic molecules. 539
B BIOCHEMISTRY ● Mtabolim is all the What is biochemistry? chemical processes that Biochemistry studies chemical processes in living cells at the molecular level. Biochemical processes, collectively known as metabolism, are take place within a living very complex and involve many chemical reactions occurring in the same place and at the same time. Some of these reactions ( anabolic organism to maintain life. reactions) produce large organic molecules from simpler organic or inorganic substances while in other reactions ( catabolic reactions), ● Aabolim is the complex molecules are broken down into smaller fragments. biosynthesis of complex A historical perspective molecules from simpler In the nineteenth century the main goals of biochemical studies were the isolation and identication of chemical substances present units that usually requires in living organisms. Progress in analytical techniques allowed more data to be collected, which eventually led to the discovery energy. of certain patterns in distribution of these substances in organisms and their possible roles in biochemical processes. These ndings ● Catabolim is the in turn stimulated more focused research and the utilization of a wide range of physicochemical methods that became available to breakdown of complex scientists in the twentieth century. As more complex molecules and reactions became known, the focus of biochemistry gradually shifted molecules in living towards the study of metabolic pathways and eventually to better understanding of the basic functions of living organisms and the organisms into simpler phenomenon of life. units that is usually accompanied by the release of energy. ● A mtabolic patwa is a biochemical transformation of a molecule through a series of intermediates (metabolites) into the nal product. What drives metabolism? Anabolic reactions increase the complexity and order of biochemical systems and thus reduce their entropy (sub-topic 15.2). Such processes cannot be spontaneous; they require energy, which is supplied by catabolic reactions or in photosynthesis is received in the form of light from the sun. Photosynthesis is the major source of energy for green plants and some bacteria. Other organisms, including humans, rely entirely on the chemical energy obtained from food by a complex set of metabolic processes known as respiration. Photosynthesis and respiration will be discussed later in this topic. The life functions of all organisms depend on a sophisticated balance between anabolic and catabolic processes in their cells, intake of nutrients, excretion of waste products, and exchange of energy with the environment. The variety of metabolic pathways allows living organisms to adapt to the constantly changing natural world. Life in turn affects the environment on both the local and global scale. Therefore a detailed understanding of metabolism is essential for all life sciences, from pharmacology and nutrition to ecology and agriculture, so biochemistry is increasingly becoming their common language. Molecules of life 2 2 The primary chemical element in all biologically electronic conguration of the outer shell (2s 2p important molecules is carbon. Its relatively small size, moderate electronegativity, and the 1 3 in the ground state and 2s 2p in the excited state) allow carbon to form up to four single or multiple 540
B.1 In T r Od u C T IO n TO BI O C h e MI s T r y covalent bonds with many elements, including inorganic compounds such as carbon dioxide to giant itself. The energies of these bonds are high enough biopolymers like proteins and nucleic acids. However, to produce stable molecules and at the same time from a virtually unlimited number of possible low enough to allow such molecules to undergo combinations of carbon atoms with other elements, various transformations. This combination of only a small set of relatively simple organic molecules stability and reactivity makes organic molecules the is particularly important for living organisms. These chemical basis oflife. molecules, composed of carbon, hydrogen, oxygen, nitrogen, and some other bioelements (table 1), The unique ability of carbon to form single and are used as building blocks for biopolymers of multiple bonds with itself allows for the formation of hierarchically increasing complexity (gure 1). molecules of any size and complexity – from simple carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur fatty acids amino acids nucleic bases sugars lipids peptides nucleotides polysaccharides proteins nucleic acids Figure 1 The hierarchy of biomolecules Macobiolmt Pctag b Micobiolmt Pctag b ma i t bo ma i t bo oxygen 65 iron 0.006 carbon 19 uorine 0.004 hydrogen 9.5 zinc 0.003 TOK nitrogen 2.8 silicon 0.002 In the study of the intermediate processes calcium 1.5 copper 4 of metabolism we have 1 × 10 to deal not with complex phosphorus 1.1 boron substances which elude 5 ordinary chemical methods, sulfur 0.25 iodine 7 × 10 but with the simple substances undergoing potassium 0.30 selenium 5 comprehensible reactions. 2 × 10 sodium 0.15 manganese Sir Frederick Gowland 5 Hopkins. 1914. chlorine 0.15 nickel 2 × 10 “ The dynamic side of magnesium 0.05 molybdenum 5 biochemistry”. In Repor t 2 × 10 total 99.8 other bioelements on the 83rd Meeting of 5 the British Association 1 × 10 for the Advancement of 5 Science. P653. 1 × 10 541 5 1 × 10 Table 1 Macro- and microbioelements in the human body
B BIOCHEMISTRY Water: Solvent, reactant, and product The most common types of biochemical reaction are nC H O → H (C H O) OH + (n 1)H O condensation, hydrolysis, oxidation, and reduction, 2 in which water plays the role of both the solvent and, 6 12 6 6 10 5 n at the same time, the reactant or product. Nearly all biopolymers form by condensation reactions that glucose amylose water release water as a by-product. For example, amylose (a component of starch, topic B.10) is produced in This reaction is reversible – in the human body, green plants by polycondensation of glucose: amylose is hydrolysed into glucose: H (C H O) OH + (n 1)H O → nC H O 2 6 10 5 n 6 12 6 amylose water glucose Worked example Up to 65% of the human body mass is composed of water, with intracellular uids and blood plasma containing 70–80 and 90–93 % Cyclodextrins are water, respectively. Biochemical reactions proceed in a highly controlled structurally similar to aqueous environment where most of the reactants, products, and amylose but the fragments catalysts (enzymes) are water soluble or form soluble complexes with of glucose in their molecules other molecules. This fact makes the chemical transformations in living form a large ring instead of organisms very different from those of traditional organic chemistry a chain. Deduce an equation (topic 10, sub-topics 20.1 and 20.2), where the reactions usually proceed for complete hydrolysis of in organic solvents and the presence of water is carefully avoided. the cyclodextrin containing six glucose residues. How The nature of biochemical reactions many molecules of water will be required to balance The reactions responsible for the synthesis and hydrolysis of peptides and this equation? proteins (sub-topics B.2 and B.7), fats and phospholipids (sub-topicB.3), nucleotides and nucleic acids (sub-topic B.8), and many other biologically Solution important molecules are very similar. Cyclic polymers do not In contrast to traditional organic reactions, which often require high temperatures and long reaction times, and almost never give products have terminals, so the with 100% yield, biochemical reactions usually proceed very fast and with near quantitative yields at body temperature (around 310K formula of the cyclodextrin in humans). The reason for this striking difference is the action of enzymes – highly specic and efcient biological catalysts. Enzymes and is (C H O ) . Because each enzymatic catalysis will be discussed in sub-topics B.2 and B.7. 6 10 5 6 Life and energy glucose residue needs one Oxidation and reduction of organic substances in living organisms proceed stepwise and involve a series of metabolites that transfer and oxygen and two hydrogen store energy in chemical bonds of their molecules. As has already been explained (sub-topic 9.1), redox processes can be described in terms of atoms to produce glucose, oxidation numbers, transfer of electrons, or combination with certain elements (oxygen and hydrogen). In the aqueous environment of the the number of water organism reactions involving protons or water are prevalent and so biochemists rely on the third method, occasionally referring to electron molecules in the balanced transfer when half-equations are discussed. equation will be also six: (C H O ) + 6H O → 6C H O 6 10 5 6 2 6 12 6 cyclodextrin glucose ● O xiatio is the loss of two hydrogen atoms or the gain or of an oxygen atom. ● O xiatio is the loss of electrons. ● rctio is the gain of two hydrogen atoms or the ● rctio is the gain of electrons. loss of an oxygen atom. 542
B.1 In T r Od u C T IO n TO BI O C h e MI s T r y Owing to the nature of organic molecules, hydrogen atoms are usually The stepwise lost or gained in pairs, and a single oxygen atom is added to or removed from a molecule at each metabolic step. In the following two reactions nature of both ethanol and ethanal are oxidized – ethanol loses two hydrogen atoms while ethanal gains an oxygen atom: biochemical CH CH OH + [O] → CH CHO + H O reactions 3 2 3 2 The energy liberated when substrates undergo air ethanol ethanal oxidation is not liberated in one large burst, as CH CHO + [O] → CH COOH was once thought, but is released in a stepwise 3 3 fashion. The process is not unlike that of locks ethanal ethanoic acid in a canal. As each lock is passed in the ascent from The oxidation of ethanol and ethanal can be also described in terms of a lower to a higher level a electron transfer using half-equations, in which both molecules lose electrons: certain amount of energy is expended. CH CH OH → CH CHO + + + 2e 3 2H Eric Glendinning Ball. 1942. “Oxidative mechanisms 3 2 in animal tissues”. In A CH CHO + HO → CH COOH + + + 2e symposium on respiratory 3 2 3 2H enzymes. P22. The next examples show two reduction processes – in the rst ethanal gains two hydrogen atoms and in the second hydrogen peroxide loses an oxygen atom: CH CHO + [2H] → CH CH OH 3 3 2 HO → H O + [O] 2 2 2 The reduction of ethanal and hydrogen peroxide can be presented as half-equations in which both molecules gain electrons: CH CHO + + + 2e → CH CH OH 3 2H 3 2 HO + + + 2e → 2H O 2H 2 2 2 Worked example ● Pototi is the biosynthesis of organic In the human body, a series of metabolic processes can lead to the molecules from carbon following summary equation: dioxide and water using the CH C(O)COOH + CH CH(OH)CH → CH CH(OH)COOH + CH C(O)CH 3 3 3 3 3 3 energy of light. 2-oxopropanoic propan-2-ol 2-hydroxypropanoic propanone acid (pyruvic acid) acid (lactic acid) rpiatio is the metabolic ● processes that release Which of the two reactants, 2-oxopropanoic acid or propan-2-ol, is energy from nutrients oxidized and which one is reduced? Deduce redox half-equations for both processes using protons or water where necessary. consumed by living organisms. Solution ● Aobic piatio is Propan-2-ol (C H O) has two more hydrogen atoms than propanone 3 8 the reverse process of (C H O), so propan-2-ol is oxidized. If one reactant undergoes oxidation, 3 6 photosynthesis, in which another reactant (in our case, 2-oxopropanoic acid) must undergo carbon dioxide and water reduction. Indeed, 2-oxopropanoic acid gains two hydrogen atoms and are formed from organic forms 2-hydroxypropanoic acid. Because all the reactants and products molecules and oxygen. are neutral molecules, the number of lost or gained electrons in each ● Aaobic piatio is half-equation must be equal to the number of protons: the catabolism of organic CH CH(OH)CH → CH C(O)CH + + + 2e (reduction) 2H 3 3 3 3 compounds that does not propan-2-ol propanone involve molecular oxygen + as an electron acceptor. 2H CH C(O)COOH + + 2e → CH CH(OH)COOH (oxidation) 3 3 2-oxopropanoic 2-hydroxypropanoic acid acid 543
B BIOCheMIsTry Figure 2 Photosynthesizing blue-green algae (left) and green leaf cells containing chloroplasts (right) Photosynthesis The process of photosynthesis begins when light When sunlight is not available this reduction can be energy is absorbed by chlorophylls (sub-topic B.9). reversed, and the energy needed for life functions In plants chlorophylls are held inside organelles can be produced by the oxidation of glucose by called chloroplasts. The absorbed light energy oxygen in a process called aerobic respiration: is used in a series of anabolic reactions that ultimately leads to the reduction of carbon dioxide CH O + 6O 6CO + 6H O + energy into energy-rich organic molecules such as glucose, and the release of oxygen: 6 12 6 2 2 2 glucose light Aerobic respiration also takes place in the cells of humans and other animals, who cannot 6CO + 6H O CH O + 6O utilize sunlight and are completely dependent 2 on the chemical energy of nutrients supplied by 2 2 6 12 6 photosynthesizing green plants. glucose Photosynthesis, respiration, and the atmosphere Photosynthesis and respiration are responsible for the global balance of oxygen and carbon dioxide. Nearly all the oxygen in the Earth’s atmosphere and oceans is a by-product of photosynthesis, the process carbon dioxide emissions from vehicles and factories animal plant respiration respiration organic carbon in living organisms decay dead organisms and respiration in soil waste products organisms and roots fossils and fossil fuels taken up by Figure 3 The carbon cycle oceans 544
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